Why is my check-engine light on?

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My car’s check-engine light came on for a problem with the oxygen sensor, which I purchased and replaced, but it just came back on again. What should I do?

Q. My car’s check-engine light comes on, then goes off, and now it is back on. I had a test done at local auto shop. It showed a code of PO138, which indicates a problem with the oxygen sensor. I purchased and replaced the oxygen sensor, but the light never went out. I contacted the mechanic who services my vehicle, and he said codes don’t always indicate what’s wrong. What should I do?


A. Computer fault codes can sometimes be misleading even to the best technician. There is a specific troubleshooting procedure that needs to be followed for correct diagnosis and repair. The sensor will never work correctly if there is a problem with the wiring to and from the sensor. A technician will usually check for proper voltage and grounds to the sensor. Even a new sensor with a faulty ground circuit will not function properly.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE certified master technician. Email your car question to jpaul@aaanortheast.com.