Emissions · Severity: Low
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
The downstream oxygen sensor shows the catalytic converter is not storing oxygen efficiently.
Common symptoms
- Engine light
- Failed emissions test
- Sometimes rattling or sulphur smell
Likely causes
- Worn catalytic converter
- Exhaust leak before the rear sensor
- Faulty downstream O2 sensor
- Engine misfire or rich running
First test to run
Compare upstream and downstream O2 sensor activity at 2,500 rpm; a downstream signal mirroring the upstream indicates a weak catalyst.
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