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Q: After sensor replacement, 2nd diagnostic revealed P0139

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Two days ago I took my car to the dealership because the check engine light was on. They told me it was the oxygen sensor and they replaced it. The next day, the same warning light was back on. I went to a local auto store where they ran a check engine diagnosis. The code read: "P0139 bank 1 sensor 2 o2 electrical circuit was responding slower than required for a predetermined period of time." I will return to the dealer to get this problem addressed. My car has approx. 57,000 miles and so far no serious problems. Do you think that the problem could be the catalytic converter or perhaps the wiring that connects the sensors?

A: No, it is not the catalyst. The P0139 code ...

No, it is not the catalyst. The P0139 code is for the O2 sensor before the catalyst. The code is illuminated when the engine control module is testing the O2 sensor and it cuts the fuel to see if the O2 sensor detects a lean condition within 5 seconds. If it does not, then it sets the code.

This can be caused by the evaporative vent solenoid allowing fuel vapor into the engine when it was commanded closed for the O2 sensor test. If fuel vapor leaks past the valve then the engine will get excessive fuel causing the O2 to detect it as a problem. Have the EVAP vent valve checked for leaks and replace as needed. If you need help having this looked at, a certified professional from YourMechanic can inspect the code further and determine exactly what needs repair.

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