The upstream Bank 1 O2 sensor exhaust sample is faulty (e.g., a leak at the sensor).
SYMPTOMS
- Check-engine light
- Erratic fuel trims
LIKELY CAUSES
- Exhaust leak near the sensor
- Loose sensor
- Failing sensor
DIAGNOSIS STEPS
- Inspect for exhaust leaks at the sensor
- Check sensor seating
- Replace sensor if faulty
RELATED PARTS
- Upstream O2 sensor