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Q: Problem with Check Engine light in my dashboard.

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I went to a mechanic and he said the symptom is that the MIL is on with P0139 secondary heated oxygen sensor 2 but when he checked the sensor it was working fine and told me that the probable cause is the ECM/PCM misinterpret the secondary heated oxygen sensor output and sets DTC P0139. He said I should update my software. How much will it cost me to update my software? And if I choose not to update my software and leave the light on is that gonna be okay? The mechanic turned off the light but after a week the light came back on my dashboard. Thanks.

My car has 202000 miles.
My car has an automatic transmission.

Hi and thanks for your question. The code P0139 means that the secondary oxygen sensor on bank 1 (the sensor after the catalytic converter) is not damaged, but rather the engine air fuel ratio is not being adjusted by the oxygen sensor signal. Check the connections to the O2 sensor and clean off the connections to the wiring harness from any debris or moisture. This symptom could be caused by the MAF (Mass Air Flow) sensor failing. I recommend replacing the MAF sensor first before anything. This causes the most issues with fuel trim in the engine. The O2 sensor is doing its job by signaling the computer that the fuel mixture is out of range. If you need assistance in troubleshooting, I recommend seeking out a professional, preferably a YourMechanic professional to help you with your vehicle.

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