Hi guys. I recently got a new exhaust put on my car. I have a BBK H-pipe with high flow cats and a flowmaster catback. Ever since I got the new exhaust the check engine light comes on every time I drive the car. I checked the codes and I got memory code 41 and 91. Both codes have to do with the O2 sensors on the right and left side indicating the system is lean. When I had the exhaust put on I replaced the O2 sensors on the right side. I have check everything with the O2 sensors to make sure they are in correctly and connected. I have even switched them around left and right but I still get the check engine light. Has anyone else had this problem?
engine codes
- Thread starter habiv
- Start date
whtpny
New Member
Zero_chance
Founding Member
I got a code 41 so I replaced both O2s and now Im getting 41 and 91. I just havent had the time to check it all out. My new clutch is just sitting at work collecting dust. Sweating over other peoples cars all day really makes me just wanna go home after work.
whtpny-I just had the car up on the lift and I sorted out all of the leaks. So as far as I can see there are no leaks anywhere with the exhaust.
Zero_chance-Let me know if you figure anything out. It sounds like you are having the same problem I am.
One thing I have noticed is that the check engine lite does not come on right when I start the car. I feel like it tends to come on if I get on the gas a bit. If I am just putting around town it takes longer for the light to come on. Anyone else have any ideas?
Zero_chance-Let me know if you figure anything out. It sounds like you are having the same problem I am.
One thing I have noticed is that the check engine lite does not come on right when I start the car. I feel like it tends to come on if I get on the gas a bit. If I am just putting around town it takes longer for the light to come on. Anyone else have any ideas?
Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more 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
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 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
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more 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
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 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
Zero_chance
Founding Member
Habiv what are the conditions of your O2s? I think my problem is crap in the connector and it gummed up the connection so the signal return is crap (0 volts or close to it). Im gonna hok up my breakout box to confirml. Hence always lean. Im planning on buying a new O2 sensor harness anyways cuz I have a feeling it might be playing part of the problem. Ill let you know what I find out when I start putting some time into it. This wekend I plan to dedicate to my clutch getting done. Probably gonna get stalled when I get to the flywheel cuz I dont know if the local machine shop is open on saturdays. The only good thing I got going for me is all my exhaust hardware is brand new as well as the tranny crossmember so I wont have to struggle with rusted up crap. Plus Ive been waiting to try out my Mac 19.2 volt cordless impact wrench.
Zero_chance - I recently cleaned out all the connections so i don't think there is a problem with the connection. I think the problem may be that one of the 02 sensors is old. From reading the post above by jrichker It says that as an 02 sensor gets older it gets less tollerant and slower. I think that maybe the old sensor is just not able to keep up with the new, higher amount of air. I am just going to bite the bullit and buy another new 02 sensore and see what happens. At least now I'll have two new sensors. I'll let you know what happens. Thanks for all the help guys. And have fun putting in that new clutch. that my next project 
Zero_chance
Founding Member
Thanks, hopefully I wont need luck!! I really wanted to go King Cobra on the clutch but I just dont have the extra cash up front so I had to get a stock replacement though work since I can pay them off over time out of my check. Oh well, new is better than nothing. Just like you said Habiv, might as well go new. Kinda makes me wish our cars had PIDS to look at. Its great on newer vehicles being able to hook them up to NGS or WDS and see what the voltage is at the 02s and how its switching. Let me know if that solves the problem.
Do the voltmeter trick as described in my post. You will need an analog voltmeter (one with a pointer) or a DVM with a bar gragh display. Then you will be able to see first hand if the O2 sensors are working right. A $20 voltmeter is cheaper than a $40 O2 sensor. If you get a good one, there are lots of other things you will use it for on your car.
Similar threads
