Check Engine Light is on
Trouble codes related to O2 sensor are stored
Poor fuel economy
Rough idle
Engine misfires
Hesitation or sluggish acceleration
Failed emissions test
Strong fuel smell from exhaust
Black smoke from exhaust
Unstable or fluctuating engine performance
Sensor voltage readings are abnormal
Sensor response is slow or stuck
Visible damage to the sensor or wiring
Contamination from oil, coolant, or fuel
High mileage with original sensor still installed