P0420 – Catalyst System Efficiency Below Threshold

Plain-English Explanation

Your engine control unit has detected that the catalytic converter on Bank 1 is not cleaning the exhaust gases as efficiently as expected. The upstream and downstream oxygen sensors are seeing similar readings, which suggests the catalyst isn’t doing its job.

Common Symptoms

• Check engine light on (often with no obvious driving issue).
• May fail an emissions test.
• Occasional reduced fuel economy.

Typical Causes

• Ageing or damaged catalytic converter.
• Exhaust leaks before or around the catalyst.
• Faulty downstream O2 sensor (sensor after the cat).
• Underlying mixture issues (misfires, running rich/lean) that have stressed the cat.

What To Check First

1. Check for any other stored codes (e.g. misfire or fuel trim codes) and fix those first.
2. Inspect the exhaust for leaks, cracks or poor welds near the catalytic converter.
3. Visually inspect wiring to the downstream O2 sensor.
4. If available, compare live data from the upstream vs downstream O2 sensors.

P0420 on VW/Audi Platforms

On VAG cars, P0420 can follow long-term mixture issues or misfires that have overheated the catalyst. Always fix any underlying mixture or ignition problems first, verify there are no exhaust leaks ahead of the catalyst, and then reassess before replacing the converter itself.