Emissions codes (P04xx)
Catalytic converter, EGR, and exhaust aftertreatment faults — often the only symptom is the light itself, plus a failed emissions test.
Important: Content here is general information only, not professional mechanical advice or a diagnosis for a specific vehicle. When in doubt, have a qualified mechanic assess it promptly.
- P0420: Catalyst System Efficiency Below Threshold (Bank 1) — Your car's computer has decided the catalytic converter on bank 1 (the side of the engine containing cylinder 1) is no longer cleaning exhaust gases as effectively as it should.
- P0430: Catalyst System Efficiency Below Threshold (Bank 2) — The catalytic converter on bank 2 (the side of the engine opposite cylinder 1, on V6/V8 engines) is no longer cleaning exhaust gases as effectively as it should. It is the bank-2 twin of P0420.
- P0401: Exhaust Gas Recirculation Flow Insufficient Detected — The exhaust gas recirculation (EGR) system is not flowing enough exhaust gas back into the engine. The EGR system lowers combustion temperatures to cut nitrogen-oxide emissions.