Catalyst System Efficiency Below Threshold (Bank 1)
What it means
The catalytic converter on bank 1 is not cleaning exhaust gases as well as expected, based on the two oxygen sensors before and after it.
Symptoms
Usually none beyond the check-engine light; sometimes a faint sulphur smell or failed emissions test.
Common causes
Worn-out catalytic converter, faulty downstream oxygen sensor, exhaust leak, or lingering misfire/rich condition.
Recommended action
Rule out oxygen-sensor and exhaust-leak causes before replacing the (expensive) catalyst.
Related codes
FAQ — P0420
What does code P0420 mean?
Catalyst System Efficiency Below Threshold (Bank 1). The catalytic converter on bank 1 is not cleaning exhaust gases as well as expected, based on the two oxygen sensors before and after it.
What are the symptoms of P0420?
Usually none beyond the check-engine light; sometimes a faint sulphur smell or failed emissions test.
What causes P0420?
Worn-out catalytic converter, faulty downstream oxygen sensor, exhaust leak, or lingering misfire/rich condition.
What should I do about P0420?
Rule out oxygen-sensor and exhaust-leak causes before replacing the (expensive) catalyst.
General reference based on the SAE J2012 standard. Exact causes for your vehicle may differ — when in doubt, consult a workshop.
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