Upstream Bank 1 sensor voltage stuck low (lean / short to ground).
SYMPTOMS
- Check-engine light
- Economy loss
- Drivability issues
LIKELY CAUSES
- Sensor failure
- Wiring short
- Exhaust/vacuum leak
- Fuel mixture issue
DIAGNOSIS STEPS
- Read sensor voltage
- Check wiring for short
- Verify fuel trims and exhaust integrity
RELATED PARTS
- Upstream O2 sensor