P0420 fault code explained for UK drivers: what catalyst efficiency below threshold means, common causes, useful checks and when a catalytic converter may not be the first part to replace.
Diagnose soon — especially before an MOT
P0420 means the engine control unit has judged the catalytic converter on Bank 1 to be operating below the expected efficiency threshold. It is an emissions-system code, but it does not by itself prove that the catalytic converter must be replaced.
No. The code describes what the ECU has detected, not which part has definitely failed. A worn catalyst is one possibility, but exhaust leaks, oxygen-sensor behaviour and upstream engine faults can also make catalyst monitoring fail. Replacing the converter before checking those areas can turn an expensive repair into an expensive misdiagnosis.
A sensible diagnosis looks for related fault codes, misfires, fuel-trim problems and exhaust leaks before comparing upstream and downstream oxygen-sensor behaviour. If the engine has been running rich, burning oil or misfiring, the reason for that should be corrected before judging the catalyst.
An illuminated engine malfunction indicator can be an MOT major defect on vehicles where the lamp is within the scope of the test. Emissions performance also matters independently, so clearing a code shortly before the test does not repair the underlying cause.
Costs depend on whether the fault is a sensor, leak, wiring issue or the catalyst itself. Catalytic converters also vary greatly in price by vehicle, and some cars have more than one converter or difficult access.
AutoAid can consider P0420 alongside the vehicle, symptoms and follow-up answers. That is more useful than treating the code as a replacement-parts instruction.
Start guided diagnosisSAE J2012 defines the diagnostic trouble-code format used for generic OBD codes. AutoAid's live reference layer uses the open OBDex generic-code dataset. Manufacturer-specific codes and test procedures should be checked against the vehicle maker's technical information.
AutoAid provides AI-assisted guidance only and never a guarantee. All faults, urgency ratings and prices are estimates — a physical inspection by a qualified mechanic may still be required.