From Ross-Tech Wiki
16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold
- Leakage in Intake and/or Exhaust System
- Catalyst faulty
- Oxygen Sensor(s) faulty
- Oxygen Sensor(s) Control faulty
- Check Intake and Exhaust System for Leaks
- Check Catalyst
- Check Oxygen Sensor(s)
- Check Oxygen Sensor(s) Control
- Perform Oxygen Sensor(s) Aging Check
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