Bad Oxygen Sensor=Running Lean?

[Dr_Shakalu]

Hey everyone. Still trying to pass California smog. Replaced EGR Solenoid and EGR Valve. Almost passing. NOX at 15 mph still just a little bit high. Got a code P0155 heater circuit. Haven't had time to check the circuit and oxygen sensor. Did check the fuse and it was still okay. The smog tech told me that it seems like the car is running lean. Can a faulty heater circuit or oxygen sensor cause the engine to run lean?

 

[ecss]

Under certain conditions when you turn the ignition On the PCM does a test and monitors the output voltage of the o2 sensors. When the o2 sensor is cold the voltage is around 450 millivolts and is not usable. As the o2 sensor is heated by the filament it reaches a temperature where the sensor voltage starts swinging above and below 450 millivolts. This shouldn't take more than a minute or so. If the voltage doesn't change the PCM sets P0155 indicating either the heater circuit is bad or the part of the o2 sensor that senses the oxygen content is bad.

Even if the heater circuit was bad, the o2 sensor will eventually heat up because of the exhaust temperature and it should provide a valid output. In your case it appears the o2 sensor is bad and most likely the heater circuit is working. You would need a scan tool or scanner software to monitor the o2 sensor voltage. When working properly the voltage swings back in forth from around .9 to .1 volts.

Replace the o2 sensor and that will probably fix your problem.

 

[Dr_Shakalu]

Under certain conditions when you turn the ignition On the PCM does a test and monitors the output voltage of the o2 sensors. When the o2 sensor is cold the voltage is around 450 millivolts and is not usable. As the o2 sensor is heated by the filament it reaches a temperature where the sensor voltage starts swinging above and below 450 millivolts. This shouldn't take more than a minute or so. If the voltage doesn't change the PCM sets P0155 indicating either the heater circuit is bad or the part of the o2 sensor that senses the oxygen content is bad.

Even if the heater circuit was bad, the o2 sensor will eventually heat up because of the exhaust temperature and it should provide a valid output. In your case it appears the o2 sensor is bad and most likely the heater circuit is working. You would need a scan tool or scanner software to monitor the o2 sensor voltage. When working properly the voltage swings back in forth from around .9 to .1 volts.

Replace the o2 sensor and that will probably fix your problem.

Thank you. I ordered 2 oxygen sensors to replace both of the front ones. Picking them up tomorrow. Should I change both or only the one that's not working?

 

[Hib Halverson]

Another issue which is likely affecting you and which affects many others with late C4s trying to get though the Smog Check is that often, EGR is calibrated to enable only above 17-mph. Obviously, with the low-speed part of the enhanced test done at 15, there not EGR and, in some cases, you end up with EGR content in the exhaust too high to pass.

To find out of that's your problem, you need a scan tester which can read the EGR state in its data list, then you need to road test at slow speed accelerating gently from about 12-mph to about 20-mph and not the speed at which EGR enables. If EGR is enabling above 15-mph, the solution is to have someone who can calibrate a 96, change the cal such that EGR is enabled at 14 mph rather than 17. That way, the EGR system will be working and NOx will go way down.

I do not know for sure if the 96 calibration is like that but it was on my 95 and the solution above is how I fixed it.

If the engine light is on or comes on because of a DTC P0155 (heater circuit, rt. ft. sensor), you'll never pass because, if the MIL is on you flunk automatically. Also, after codes are cleared, all your readiness flags but one must be set or you still flunk.

Yes, the code for heater circuit can set because the sensor is bad, but it also can set if there is a problem with the heater circuity. You can try changing sensors but that might not fix it. Best bet is to diagnose DTC P0155 per the FSM which will help you decide if it's the sensor or the wiring.

As for the tech telling you the engine runs like it's lean...if it is, in fact, lean, it's possible that's because of bad front sensors. It could, also, be that the engine truly is running lean. If so, you need to address that before you try and test because, if it is running lean, that raises combustion temperatures and that, in turn, increases NOx.

So actually, with respect to flunking due to NOx you have one of two or maybe both problems 1) EGR system disabled until 17-mph and/or 2) engine lean making for high combustion temperature and high EGR.

 

[Dr_Shakalu]

Another issue which is likely affecting you and which affects many others with late C4s trying to get though the Smog Check is that often, EGR is calibrated to enable only above 17-mph. Obviously, with the low-speed part of the enhanced test done at 15, there not EGR and, in some cases, you end up with EGR content in the exhaust too high to pass.

To find out of that's your problem, you need a scan tester which can read the EGR state in its data list, then you need to road test at slow speed accelerating gently from about 12-mph to about 20-mph and not the speed at which EGR enables. If EGR is enabling above 15-mph, the solution is to have someone who can calibrate a 96, change the cal such that EGR is enabled at 14 mph rather than 17. That way, the EGR system will be working and NOx will go way down.

I do not know for sure if the 96 calibration is like that but it was on my 95 and the solution above is how I fixed it.

If the engine light is on or comes on because of a DTC P0155 (heater circuit, rt. ft. sensor), you'll never pass because, if the MIL is on you flunk automatically. Also, after codes are cleared, all your readiness flags but one must be set or you still flunk.

Yes, the code for heater circuit can set because the sensor is bad, but it also can set if there is a problem with the heater circuity. You can try changing sensors but that might not fix it. Best bet is to diagnose DTC P0155 per the FSM which will help you decide if it's the sensor or the wiring.

As for the tech telling you the engine runs like it's lean...if it is, in fact, lean, it's possible that's because of bad front sensors. It could, also, be that the engine truly is running lean. If so, you need to address that before you try and test because, if it is running lean, that raises combustion temperatures and that, in turn, increases NOx.

So actually, with respect to flunking due to NOx you have one of two or maybe both problems 1) EGR system disabled until 17-mph and/or 2) engine lean making for high combustion temperature and high EGR.

Thanks for the advice. After I changed the EGR valve and EGR solenoid, I changed both front oxygen sensors (passenger side was a PITA). I also remembered that last year, I got a P1441 code for the evap purge solenoid. I changed it but I got the code again about 4 months ago. I erased the fault and haven't received any codes but right after I changed the oxygen sensors, I got the P1441 again. Erased the code and on the way to the smog shop I stopped at Oreilly's to pick up some vacuum hoses. Decided to check on the evap purge solenoid just in case it was still a problem. Took out the solenoid and noticed that it wasn't holding vacuum. Also, the vacuum hoses connected to it were leaking. $20 for the solenoid (luckily they had it in stock) and vacuum lines, 10 minutes in the parking lot and I was ready to smog. Passed the smog test with flying colors.