A malfunctioning upstream oxygen sensor provides incorrect data to the PCM, causing it to command an overly rich fuel mixture. Confirm this by monitoring live data on a scan tool. A healthy sensor should oscillate rapidly between 0.1V (lean) and 0.9V (rich). If the voltage remains locked above 0.5V-0.7V while the engine is at operating temperature, the sensor is failing to report the mixture accurately.
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