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Q: Check engine, traction off lights are showing

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I purchased my car used, and the check engine and traction off lights are on. I checked on the code scanner, it showed: knock sensor. After fixing that, then O2 sensor codes starting show. What else do I need to fix?

The Check Engine Light is likely still on due to the oxygen sensor faults. The unfortunate thing is that engine computers sometimes cannot detect another problem if they are dealing with one already. That may be why it didn’t have oxygen sensor faults at first, but set the faults after you fixed the other problem.

The best thing you can do now is to diagnose the issue with the oxygen sensors, as it may not be fixed by replacing the sensors. You also may need to scan the ABS control module, as there may be faults stored in that which are causing the traction control light to be on. If this sounds like more work than you are up for, I suggest contacting a professional technician to have them diagnose the real cause of the check engine light.

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