Fault in the downstream (post-catalyst) bank 1 oxygen-sensor circuit. The downstream sensor monitors catalyst efficiency; its signal should be relatively steady. A fault here is usually the sensor or wiring rather than a mixture problem.
SYMPTOMS
- Check-engine light
- Usually no drivability symptoms
- Possible catalyst-monitor readiness failure
LIKELY CAUSES
- Aged/contaminated downstream O2 sensor
- Wiring or connector fault
- Exhaust leak near the sensor
- Heater circuit fault
DIAGNOSIS STEPS
- Read downstream O2 live data
- Inspect connector and wiring
- Check for exhaust leaks
- Replace the sensor if faulty
RELATED PARTS
- Downstream O2 sensor (Bank 1 Sensor 2)