P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
Moderate severityWhat it means
The downstream oxygen sensor shows that the catalytic converter on bank 1 is no longer cleaning the exhaust as efficiently as it should. The converter may be aging, or another fault — like misfires or a bad sensor — may be skewing the reading.
Common symptoms
- Often no symptom other than the check engine light
- Slight loss of power or fuel economy
- Sulphur (rotten egg) smell from the exhaust
- Failed emissions inspection
Likely causes
- Aged or failing catalytic converter
- Faulty downstream (post-cat) oxygen sensor
- Exhaust leaks before or around the sensor
- Misfires or rich/lean running that damaged the converter
- Oil or coolant entering the exhaust
What to check first
- Fix any other stored codes first (misfire and fuel-trim codes especially)
- Compare upstream and downstream O2 sensor signals in live data — a downstream signal that switches like the upstream one points to a worn converter
- Inspect the exhaust for leaks
- Review long-term fuel trims
Can you keep driving?
Generally drivable, but address it soon: the underlying cause can worsen, and the car is likely to fail an emissions test.
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