Originally Posted by domano 68
After cleaning the maf, tb, new plugs, and new pcv my sisters 99 still idles rough at lights. She has a cel w a p0138 code. I am not certain but is this the rear most u2 sensor? Also would a bad u2 sensor cause rough idle at lights( does not stall out)?
I have done an extensive search therefore the remaining questions. TIA!
If you read the 'Check Engine Light' sticky you could of confirmed that P0138 (0510) does pertain to the downstream O2 sensor after the CAT. This is the info it contains regarding that cel code:
Diagnostic Trouble Code 0510 points to a problem with the Rear Heated Oxygen Sensor. This sensor is mounted in the exhaust stream, just downstream of the Catalytic Converter. When the two Front Heated Oxygen Sensors are operating properly the Rear Heated Oxygen Sensor is not used for engine control operation. It is used to monitor the condition of the Catalytic Converter. If either or both Front Sensors fail the Engine Control Module uses the signal from the Rear Sensor to maintain a correct fuel/air mixture.
DTC 0510 is detected when the signal from the Rear Heated Oxygen Sensor does not reach the expected maximum voltage threshold value. The possible causes include ...
- Harness or connectors (the sensor circuit is open or shorted.)
- Rear Heated Oxygen Sensor
- Fuel pressure
- Fuel injectors
- Intake air leaks
Since you're getting a rough idle I would check the injectors, FPR, and for a vacuum leak.
A bad downstream O2 sensor has no effect on idle, but a faulty upstream O2 sensor would effect it.