Q: Check Engine light; upstream O2 sensor

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The Check Engine light is on again, and the code says it's the upstream O2 sensor.

Hi there. If you have a code for the upstream O2 sensor then that is the most likely problem. Knowing the code would help me to be able to tell for sure what is going on. If you replaced the sensor and the same code returned then you may have used the wrong type/brand of sensor, or there may be a problem with the fuel mixture. A vacuum leak or leaking fuel injector can cause an O2 sensor code. If the sensor is new then a mechanic would typically install a computer scan tool in order to check the fuel mixture to see what is going on.

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