Engine 93 Mustang GT, Code 41/91, KOER Cylinder ID shows 6 and not 8

79MercCapri93

Member
May 12, 2012
71
6
8
30
Hello everyone,

I have been jumping around multiple forums for the past 2-3 weeks now trying to gather as much information as possible and could not find some answers to a few questions. I have a 93 Mustang GT, 5 speed, with CAI, BBK Shortys, E-303, 70MM T/B and spacer, 70MM Mass air with correct sensor, electric fan, 3g ALT, and 3.73's with correct speedometer gears.Almost forgot, off-road h-pipe and flow-master dumps, as well as the SMOG pump has been deleted and the solenoids are in place but capped off. I also still retain the charcoal canister checked the solenoid to make sure it was holding pressure and no leaks from vacuum lines.

I started driving my car as a DD since my truck only gets 12MPG and my car has been hitting 22-24MPG consistently and my drive is 58 miles each way. I work shift work from the afternoon to midnight and my car runs great on the way to work without issue, but when drive home around midnight, as soon as i hit the toll booth and the bay bridge, without fail my CEL comes on and i have a 41/91 (its about 25-30 miles to get from work to the tolls/bridge). The CEL will stay on until i restart the car or reset with my scan tool and probably once or twice it came on again after getting gas on my drive home after the initial CEL.

I made a homemade smoke tester to check for vacuum leaks and the only smoke i have found thus far is coming out of the EGR by the orange rubber? material after about 4-5 minutes of applying smoke.

I also started looking for only loose connections for the HEGO orange wire and find it tight on the back of the cylinder head, and plan to smell the vacuum line going to the FPR for any signs of fuel.

Well, last night i decided to leave the car running and pull codes just in case there were any that were not triggering as a continous and would go away when i turned off the ignition. During the KOER test, the cylinder id showed 6 and not 8 cylinders and 41/91.

This morning when i reset the codes and did a KOEO and then a KOER test, it showed 6 cylinders again and no 41/91 (which only appears to be on my way home, and every time its colder during the drive home then during the day). I am checking again tomorrow or tuesday, as well as pulling plugs. I ordered new wires and a rotor and cap since they about 10 years old. When this cylinder id test is done, what exactly is occuring? in order for the cylinders to be identified? Could this be either no spark OR no fuel?

Any thoughts? I feel as though it may be safe to assume that the cylinder ID and codes 41/91 may be linked, as the car would be getting excess fuel not getting burned off. But i would think the car would run horrible? or perhaps because i just drive slow and steady at 55MPH during the night time im not noticing poor engine performance. (not sure if it matters but thats without cruise control since my C/C fluxuates 2-3 mph non stop when its on)

I have also reviewed jricker's posts several times on 41/91 and will be following those as well, but it seems my "symptoms" have not really fallen in line with anyone elses as of yet
 
Last edited:
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79MercCapri93

Member
May 12, 2012
71
6
8
30
Hello everyone,

I have been jumping around multiple forums for the past 2-3 weeks now trying to gather as much information as possible and could not find some answers to a few questions. I have a 93 Mustang GT, 5 speed, with CAI, BBK Shortys, E-303, 70MM T/B and spacer, 70MM Mass air with correct sensor, electric fan, 3g ALT, and 3.73's with correct speedometer gears.Almost forgot, off-road h-pipe and flow-master dumps, as well as the SMOG pump has been deleted and the solenoids are in place but capped off. I also still retain the charcoal canister checked the solenoid to make sure it was holding pressure and no leaks from vacuum lines.

I started driving my car as a DD since my truck only gets 12MPG and my car has been hitting 22-24MPG consistently and my drive is 58 miles each way. I work shift work from the afternoon to midnight and my car runs great on the way to work without issue, but when drive home around midnight, as soon as i hit the toll booth and the bay bridge, without fail my CEL comes on and i have a 41/91 (its about 25-30 miles to get from work to the tolls/bridge). The CEL will stay on until i restart the car or reset with my scan tool and probably once or twice it came on again after getting gas on my drive home after the initial CEL.

I made a homemade smoke tester to check for vacuum leaks and the only smoke i have found thus far is coming out of the EGR by the orange rubber? material after about 4-5 minutes of applying smoke.

I also started looking for only loose connections for the HEGO orange wire and find it tight on the back of the cylinder head, and plan to smell the vacuum line going to the FPR for any signs of fuel.

Well, last night i decided to leave the car running and pull codes just in case there were any that were not triggering as a continous and would go away when i turned off the ignition. During the KOER test, the cylinder id showed 6 and not 8 cylinders and 41/91.

This morning when i reset the codes and did a KOEO and then a KOER test, it showed 6 cylinders again and no 41/91 (which only appears to be on my way home, and every time its colder during the drive home then during the day). I am checking again tomorrow or tuesday, as well as pulling plugs. I ordered new wires and a rotor and cap since they about 10 years old. When this cylinder id test is done, what exactly is occuring? in order for the cylinders to be identified? Could this be either no spark OR no fuel?

Any thoughts? I feel as though it may be safe to assume that the cylinder ID and codes 41/91 may be linked, as the car would be getting excess fuel not getting burned off. But i would think the car would run horrible? or perhaps because i just drive slow and steady at 55MPH during the night time im not noticing poor engine performance. (not sure if it matters but thats without cruise control since my C/C fluxuates 2-3 mph non stop when its on)

I have also reviewed jricker's posts several times on 41/91 and will be following those as well, but it seems my "symptoms" have not really fallen in line with anyone elses as of yet

Please let me know if I posted this in the wrong section, I was not sure if this should be under general tech instead or not
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
26,893
2,559
224
73
Dublin GA
Hello everyone,

I have been jumping around multiple forums for the past 2-3 weeks now trying to gather as much information as possible and could not find some answers to a few questions. I have a 93 Mustang GT, 5 speed, with CAI, BBK Shortys, E-303, 70MM T/B and spacer, 70MM Mass air with correct sensor, electric fan, 3g ALT, and 3.73's with correct speedometer gears.Almost forgot, off-road h-pipe and flow-master dumps, as well as the SMOG pump has been deleted and the solenoids are in place but capped off. I also still retain the charcoal canister checked the solenoid to make sure it was holding pressure and no leaks from vacuum lines.

I started driving my car as a DD since my truck only gets 12MPG and my car has been hitting 22-24MPG consistently and my drive is 58 miles each way. I work shift work from the afternoon to midnight and my car runs great on the way to work without issue, but when drive home around midnight, as soon as i hit the toll booth and the bay bridge, without fail my CEL comes on and i have a 41/91 (its about 25-30 miles to get from work to the tolls/bridge). The CEL will stay on until i restart the car or reset with my scan tool and probably once or twice it came on again after getting gas on my drive home after the initial CEL.

I made a homemade smoke tester to check for vacuum leaks and the only smoke i have found thus far is coming out of the EGR by the orange rubber? material after about 4-5 minutes of applying smoke.

I also started looking for only loose connections for the HEGO orange wire and find it tight on the back of the cylinder head, and plan to smell the vacuum line going to the FPR for any signs of fuel.

Well, last night i decided to leave the car running and pull codes just in case there were any that were not triggering as a continous and would go away when i turned off the ignition. During the KOER test, the cylinder id showed 6 and not 8 cylinders and 41/91.

This morning when i reset the codes and did a KOEO and then a KOER test, it showed 6 cylinders again and no 41/91 (which only appears to be on my way home, and every time its colder during the drive home then during the day). I am checking again tomorrow or tuesday, as well as pulling plugs. I ordered new wires and a rotor and cap since they about 10 years old. When this cylinder id test is done, what exactly is occuring? in order for the cylinders to be identified? Could this be either no spark OR no fuel?

Any thoughts? I feel as though it may be safe to assume that the cylinder ID and codes 41/91 may be linked, as the car would be getting excess fuel not getting burned off. But i would think the car would run horrible? or perhaps because i just drive slow and steady at 55MPH during the night time im not noticing poor engine performance. (not sure if it matters but thats without cruise control since my C/C fluxuates 2-3 mph non stop when its on)

I have also reviewed jricker's posts several times on 41/91 and will be following those as well, but it seems my "symptoms" have not really fallen in line with anyone elses as of yet
The 6 cylinder ID code is something fluky that 5.0 Mustangs do. It doesn't affect how the car runs, it just spooks the owner into chasing rabbits and pulling their hair out.

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24 Aug 2018
1.) To correct the RH & LH mismatch on 91-93 5.0 Mustangs
2.) To add Tmoss’ wiring diagrams for 88-95 Mustangs


Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:

"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor, you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Computer wiring harness connector, computer side.
88243.gif


Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.gif



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor

The computer pins are 29 (RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/lt green wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor

The computer pins are 29 (RH O2 with a Gray/Lt blue wire) and 43 (LH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


94-95 5.0 Mustangs; note that the 94-95 uses a 4 wire O2 sensor.
The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. . The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH O2 sensor harness and the Dark Green/Pink wire on the computer pin 29


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 27


There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged, and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness. The exception is that the 94-95 uses a 4 wire O2 sensor.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
 

79MercCapri93

Member
May 12, 2012
71
6
8
30
Jrichker,

Thank you! I was starting to wonder if I had a fuel or spark issue! Just seems odd to me that the codes only come up at the same point but other than that I don’t see them. I slow down to 10mph for the toll and have to speed back up, but I thought if it was a true 41/92 it would be more persistent and i would see it when I hit traffic on the drive in

I’ll see if I can get out there today and run theses checks as well as checking for codes doing a KOEO and KOER w/ the cylinder balance tests!

Thanks again!
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
While you are digging around under the hood disconnect the cruise control vacuum line at the tree and plug the nipple at the tree, the cc fluctuations may be a vacuum leak in that system.
You also said you had smoke coming from the rubber on the egr valve, that may be anothe vacuum leak.
 

79MercCapri93

Member
May 12, 2012
71
6
8
30
While you are digging around under the hood disconnect the cruise control vacuum line at the tree and plug the nipple at the tree, the cc fluctuations may be a vacuum leak in that system.
You also said you had smoke coming from the rubber on the egr valve, that may be anothe vacuum leak.
Good point, I’ll do that and try testing the EGR. If I’m not mistaken I disconnect the vacuum line and cap it, and just hook my mityvac to the egr and go to 5lbs of pressure and see if the car stumbles, does that sound correct? I have another egr on a stock spacer I can check as well and a block off plate if necessary
 

79MercCapri93

Member
May 12, 2012
71
6
8
30
KOEO - no codes

KOER - 8 cylinders detected, 41/91

Cylinder balance test- 90, 90

Smoke test - smoke coming out of egr, tps and smoke inside valve cover ( honestly not sure if I should expect that?, I capped off the line going from the oil fill spout to the TB just to verify and was still seeing smoke)

I also checked my C/C while I was in the process. The check valve held pressure, but the line going from the check valve to the CC would steadily lose vacuum. So I have it capped off at the tree for the time being

Checking 02’s and orange wire tonight, wanted to earlier but ran out of time
 
Last edited:

79MercCapri93

Member
May 12, 2012
71
6
8
30
Almost forgot, When I checked the EGR doing the vacuum test the engine stumbled but the car did not die. I found a previous post by cjones

“if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 5in vacuum”

I’m assuming I just T in my vacuum gauge for all of these tests these tests? Just on different sides of the valve mounted to the fender depending on the test
 
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79MercCapri93

Member
May 12, 2012
71
6
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Good point, I’ll do that and try testing the EGR. If I’m not mistaken I disconnect the vacuum line and cap it, and just hook my mityvac to the egr and go to 5lbs of pressure and see if the car stumbles, does that sound correct? I have another egr on a stock spacer I can check as well and a block off plate if necessary

At idle, both O2 sensors were jumping between .01 and .65

Right bank O2 harness connector at 200ohm is .5

Left bank o2 harness connector at 200ohm is .5 as well
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
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Dublin GA
At idle, both O2 sensors were jumping between .01 and .65

Right bank O2 harness connector at 200ohm is .5

Left bank o2 harness connector at 200ohm is .5 as well
That looks good.

Did you clear the codes?

How to clear codes.
Clearing the codes by pressing a button on the scan tool or disconnecting the test jumper used to start the code dump does not erase the “learned settings”. All it does is erase the stored codes in memory.

You must clear the codes anytime you replace any sensor. The following tells you how and is different from the method above
Clear the computer codes by disconnecting the battery negative terminal and turn the headlights on. Turn the headlights off and reconnect the all sensors including the MAF and anything else you may have disconnected. Then reconnect the battery negative cable.. This clears all spurious codes may have been generated while troubleshooting problems. It also clears the adaptive settings that the computer "learns" as it operates. Clearing the codes does not fix the code problems, it just gives you a clean slate to start recording what the computer sees happening.

Run the car for at least 30 minutes of driving and dump the codes again to assure that you have fixed the code problem or sensor problem. This is necessary for the computer to relearn the adaptive settings that the computer uses for proper operation. The engine may run rough at first, but should smooth out as it runs for the 15-20 minute learning period.
 

79MercCapri93

Member
May 12, 2012
71
6
8
30
The 6 cylinder ID code is something fluky that 5.0 Mustangs do. It doesn't affect how the car runs, it just spooks the owner into chasing rabbits and pulling their hair out.

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 24 Aug 2018
1.) To correct the RH & LH mismatch on 91-93 5.0 Mustangs
2.) To add Tmoss’ wiring diagrams for 88-95 Mustangs


Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:

"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.


Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor, you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Computer wiring harness connector, computer side.
88243.gif


Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.gif



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor

The computer pins are 29 (RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/lt green wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor

The computer pins are 29 (RH O2 with a Gray/Lt blue wire) and 43 (LH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


94-95 5.0 Mustangs; note that the 94-95 uses a 4 wire O2 sensor.
The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. . The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.



87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH O2 sensor harness and the Dark Green/Pink wire on the computer pin 29


91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH O2 sensor harness and the Gray/Lt blue wire on the computer pin 27


There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged, and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness. The exception is that the 94-95 uses a 4 wire O2 sensor.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

Complete computer, actuator & sensor wiring diagram for 94-95 Mustangs
94-95_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
91-93_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 88-90 Mass Air Mustangs
88-91_5.0_EEC_Wiring_Diagram.gif
That looks good.

Did you clear the codes?

How to clear codes.
Clearing the codes by pressing a button on the scan tool or disconnecting the test jumper used to start the code dump does not erase the “learned settings”. All it does is erase the stored codes in memory.

You must clear the codes anytime you replace any sensor. The following tells you how and is different from the method above
Clear the computer codes by disconnecting the battery negative terminal and turn the headlights on. Turn the headlights off and reconnect the all sensors including the MAF and anything else you may have disconnected. Then reconnect the battery negative cable.. This clears all spurious codes may have been generated while troubleshooting problems. It also clears the adaptive settings that the computer "learns" as it operates. Clearing the codes does not fix the code problems, it just gives you a clean slate to start recording what the computer sees happening.

Run the car for at least 30 minutes of driving and dump the codes again to assure that you have fixed the code problem or sensor problem. This is necessary for the computer to relearn the adaptive settings that the computer uses for proper operation. The engine may run rough at first, but should smooth out as it runs for the 15-20 minute learning period.

I cleared the codes prior yesterday but will clear them again. I also disconnected the battery and computer as that was the easiest way to test for continuity last night. But when I hook everything up I’ll do the headlight method as well just to make sure. I won’t be able to drive it until Thursday though unfortunately

I’m going to try and see if I can
Grab a fuel pressure tester from Autozone today or Saturday.

Any thoughts on smoke from the EGR, TPS, or valve cover?

Thanks for all of your help!
 
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79MercCapri93

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May 12, 2012
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Fuel pressure test:

KOEO - 36psi

KOER - 32 psi

KOER w/ vacuum unhooked - 40 psi

Just running the car in the driveway after clearing EEC codes, I just get a code 91.

I also sprayed around all of the vacuum lines as well as the egr and tps and saw no fluctuation in idle.

So I’ll change out the O2’s I guess? Would running a 160 thermostat cause this issue as well? I have the correct thermostat but wanted to get any issues fixed before I swapped the upper out for an explorer intake with the correct thermostat
 

79MercCapri93

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Did some research and it seems the Bosch O2’s that’s are in the car currently are “junk” and a lot of people have problems with them, so I’ll start there and see what happens
 
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Brando30

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Did some research and it seems the Bosch O2’s that’s are in the car currently are “junk” and a lot of people have problems with them, so I’ll start there and see what happens
Was missing an 02 sensor when I purchased my car last November. Kept getting KOER 41 test with scan tool. Bought a Bosch 02 sensor, resolved engine light for a little bit. Came back on intermittently, replaced both with morotcraft sensors, no engine light.
 

79MercCapri93

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So I capped off the C/C vacuum lines, cleaned the mass air, swapped out the EGR with a different one that barely had smoke come out, and put in 2 new NGK O2 sensors. Car ran great, need to adjust idle back down, but much smoother running. No check engine like usual... until right before I got home. About 2 minutes from my house and the CEL comes on, pull codes and have 41/91 again
 
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79MercCapri93

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May 12, 2012
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So I capped off the C/C vacuum lines, swapped out the EGR with a different one that barely had smoke come out, and put in 2 new NGK O2 sensors. Car ran great, need to adjust idle back down, but much smoother running. No check engine like usual... until right before I got home. About 2 minutes from my house and the CEL comes on, pull codes and have 41/91 again
Was missing an 02 sensor when I purchased my car last November. Kept getting KOER 41 test with scan tool. Bought a Bosch 02 sensor, resolved engine light for a little bit. Came back on intermittently, replaced both with morotcraft sensors, no engine light.
I went with NGK and seemed like they were doing good, but now I’m getting 41/91 again. I’ll have to do a KOER test tomorrow and see what it says, this is driving me crazy
 

Window

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Feb 13, 2017
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i had the same issue and resolved it with a new o2 sensor harness. old one was soaked in oil, dirt, salt you name it.
 
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Mustang5L5

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If the smog tube on the back of the heads, of the ports throught he heads are clogged, air can't pass through and the o2's won't see it. Usually triggers a code 41/91.
 

79MercCapri93

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May 12, 2012
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If the smog tube on the back of the heads, of the ports throught he heads are clogged, air can't pass through and the o2's won't see it. Usually triggers a code 41/91.
I wish that was my problem right now but unfortunately my smog tube has been long gone and both sides have a bolt in them!