Code 41 and 91

Haus · Sep 9, 2008

both for KOER test. 41/91

just replaced the O2 sensors. dont know what could be the problem.

HISSIN50 · Sep 9, 2008

Things like a vac leak, dirty MAF wires, and the like can cause the issue. Search for Jrichker's 41/91 code synopsis.

Good luck.

jrichker · Sep 9, 2008

As suggested...

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

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
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. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer 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.

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

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

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.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides to that there is no carbon blocking the free flow of air to both heads.

Haus · Sep 9, 2008

i already put on new O2 sensors. needed them to pass emissions. after getting new ones the car passed with no problems

car does smell like fuel alot. mileage is pretty crappy lately

ill check the voltage of the sensors at the computer

and i hear a poping sound coming out the exhaust. no idea if that has anything to do with it

93 LX · Sep 10, 2008

jrichker said:
As suggested...
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides to that there is no carbon blocking the free flow of air to both heads.

whats the easiest way of removing the tube? I think this is my problem. I only get the code when driving a few miles, if I let the car idle down for a few minutes or get stuck in traffic the CEL goes away. All my troubles happend after I reconnected the air pump back to the Cats. The rubber hose I had there before opened up.

jrichker · Sep 10, 2008

93 LX said:
whats the easiest way of removing the tube? I think this is my problem. I only get the code when driving a few miles, if I let the car idle down for a few minutes or get stuck in traffic the CEL goes away. All my troubles happend after I reconnected the air pump back to the Cats. The rubber hose I had there before opened up.

I do not know of an easy way to remove it. This is one of the knuckle busting jobs that result in the shedding of blood.

To top it off, getting the tube cleaned out is harder than getting it off the engine.

Haus · Sep 13, 2008

so is this Orange ground 02 sensor wire in the engine bay?? can you explain more about that

jrichker · Sep 13, 2008

Haus said:
so is this Orange ground 02 sensor wire in the engine bay?? can you explain more about that

See the attached diagram and look for HEGO (Heated Exhaust Gas Oxygen) ground in the upper LH part of the diagram.

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Haus · Sep 15, 2008

ok i found the ground wire for the HEGO. its an orange wire with some sort of fabric over it but where that wire leads me its to the block. right next to the driver side head by a metal tube.

its so loose. looks to be not connected properly. and i cant get to it enought to get it connected properly. can i just cut the wire and splice another 16 gage wire to it and ground it to another spot.

jrichker · Sep 15, 2008

Best bet is to use the nearest available bolt. If you have to splice some more wire on it, solder and heat shrink the splice. How to solder like a pro - Ford Fuel Injection How To Solder Like a Pro a must read for any automotive wiring job.

Haus · Jan 12, 2009

is grounding it to the firewall be enough for the wire.

is there a test i can do to check the Hego ground is good??

machstang23 · Jan 12, 2009

I have mine grounded to the very back bolt on the drivers side of the lower intake and it seems to be working fine there. but I did locate mine there when installing my new intake so it was really easy to access with the upper intake off of the motor. and its slightly easier to access because you can reach it with a long extenstion

Haus · Jan 12, 2009

that diagram with the Computer (60 pin) is really confusing me. when i go to where the pin is the color of wire is different.

where the 29 pin is the wire color on my car is blue/gray
where the 43 wire is the color is black/red

i dont want to check anything at the moment cause the thing says a certain color and its not the color.

so im confused. im trying to check the

1992 5.0 A9p Computer

Haus · Jan 12, 2009

machstang23 said:
I have mine grounded to the very back bolt on the drivers side of the lower intake and it seems to be working fine there. but I did locate mine there when installing my new intake so it was really easy to access with the upper intake off of the motor. and its slightly easier to access because you can reach it with a long extenstion

yeah well mine was very short and i couldnt get in there and splice a new wire on it so im screwed if it has to be grounded to a bolt if i cant find one that will reach.

Haus · Jan 13, 2009

Haus said:
that diagram with the Computer (60 pin) is really confusing me. when i go to where the pin is the color of wire is different.

where the 29 pin is the wire color on my car is blue/gray
where the 43 wire is the color is black/red

i dont want to check anything at the moment cause the thing says a certain color and its not the color.

so im confused. im trying to check the

1992 5.0 A9p Computer

Anyone?? my Cpu is Dangling on my floor. still drivable but does it matter that the color of wire is different and all that matters is that its in the same pin as the 29/43??

jrichker · Jan 13, 2009

All the A9L and A9P computers had the same pinout functions. The wire harness changed in late 91 or start
of production 92, so you wire color may be different.

Diagram courtesy of Tmoss & Stang&2birds

Haus · Jan 13, 2009

you rock thanks. same pins just different color wires

Haus · Jan 13, 2009

ok pin 29 was about .3 volts and pin 43 was about .5 volts

Haus · Jan 13, 2009

how can i check and make sure the Orange ground wire is working right.

with the volts of pin 29/43 within range DOES that tell me the orange Hego ground is working properly?? even tho i have it grounded to the firewall??

so with the voltzs in range does that tell me my O2's are working properly and the wiring harness is good to go too.

so i should be looking in to something else.

HISSIN50 · Jan 13, 2009

Ohm out the HEGO ground. Your meter should show about 5 ohms.

If a reading of the O2's (pins 29/43) is a fixed voltage, there's an issue. The readings should toggle between 0.1 and 0.9 Volts about once a second. As the sensors age or degenerate, they lose amplitude and frequency. If a reading doesnt move at all, or only a few hundreths of a volt, it will trigger a CEL and a code.

Do note that you need to let the car warm up enough for the O2's to reach about 600*F, which takes 30-120 seconds. Mine do it closer to 30 seconds. Be sure your car idled for a few minutes before you do your testing (and the testing needs to be done with the car idling).

Code 41 and 91

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