P0420 fault code: what it means

P0420: Bank 1 catalytic converter is not cleaning the exhaust well enough. Check for misfires, exhaust leaks and the rear O2 sensor first.

Engine and gearbox (powertrain) · Costly if missed

Usual causes, most likely first

worn catalytic converter
aged rear O2 sensor
exhaust leak before the rear sensor
misfire or oil burning damaging the cat

P0420 by make

Audi

Catalytic converter efficiency below limit on bank 1. Rule out leaks and the rear oxygen sensor before buying a cat.

Usual causes: exhaust or intake leak; worn catalytic converter; faulty oxygen sensor; oxygen sensor wiring

BMW

Catalytic converter efficiency low, bank 1 (cylinders 1-3 on a straight six). Often follows misfires or oil burning.

Usual causes: worn catalytic converter; rear oxygen sensor; exhaust leak before the sensor; engine misfire or oil burning damaging the cat

Haval

Catalytic converter efficiency below threshold.

Usual causes: worn catalytic converter; rear O2 sensor; exhaust leak

Honda

Catalytic converter efficiency low (bank 1). On J35 V6 bank 1 is the rear converter, often killed by VCM oil burning; on 1.5 turbo, injector faults and oil dilution are linked.

Usual causes: worn or oil-contaminated catalytic converter; rear oxygen sensor; leaking fuel injectors or oil burning (1.5 turbo, VCM V6); exhaust leak before the sensor

Hyundai

Catalytic converter efficiency below threshold. On Hyundai Nu / Theta engines oil burning often kills the cat.

Usual causes: engine burning oil (Nu 2.0 / Theta II ring wear) fouling the cat; worn catalytic converter; faulty rear oxygen sensor; exhaust leak before the rear sensor

Jeep

Catalytic converter efficiency below threshold, bank 1.

Usual causes: worn catalytic converter; rear O2 sensor; exhaust leak before the sensor; misfire history damaging the cat

Kia

Catalytic converter efficiency below threshold. On Kia Nu 2.0 / Theta II often a result of oil burning.

Usual causes: engine burning oil and poisoning the cat (Nu 2.0, Theta II; piston ring recalls on Seltos/Soul 2.0); worn or overheated catalytic converter; faulty rear oxygen sensor; exhaust leak before the rear sensor

Lexus

Catalytic converter efficiency below threshold (bank 1).

Usual causes: worn or contaminated catalytic converter; exhaust leak before the rear sensor; faulty air-fuel ratio sensor; misfire damaging the cat

Mercedes-Benz

Catalytic converter efficiency below threshold, bank 1. On Mercedes, check for a split or worn cat and O2 sensor faults.

Usual causes: worn or split catalytic converter; faulty rear O2 sensor; exhaust leak before the sensor

MG

Catalytic converter efficiency below threshold.

Usual causes: worn catalytic converter; rear O2 sensor; exhaust leak

Mini

Catalytic converter efficiency low. Often follows misfires or oil burning.

Usual causes: worn catalytic converter; rear oxygen sensor; exhaust leak before the sensor; misfire or oil burning damaging the cat

Mitsubishi

Catalytic converter on bank 1 not cleaning the exhaust well enough.

Usual causes: worn catalytic converter; faulty rear O2 sensor; exhaust leak before the rear sensor; misfire or oil burning damaging the cat

Nissan

Catalytic converter (bank 1) efficiency low. On QR25 engines oil burning is a common reason the cat fails early.

Usual causes: worn or oil-fouled catalytic converter; engine burning oil (QR25); rear oxygen sensor fault; exhaust leak before the rear sensor

Skoda

Catalytic converter efficiency below limit on bank 1. Rule out leaks and the rear oxygen sensor before buying a cat.

Usual causes: exhaust or intake leak; worn catalytic converter; faulty oxygen sensor; oxygen sensor wiring

Subaru

Catalytic converter on bank 1 is not cleaning the exhaust well enough. On FB25 oil burning often poisons the cat.

Usual causes: worn or oil-contaminated catalytic converter; faulty rear O2 sensor; exhaust leak before the rear sensor; failing front A/F sensor or misfire

Suzuki

Catalytic converter efficiency below threshold.

Usual causes: worn catalytic converter; rear O2 sensor; exhaust leak

Toyota

Catalytic converter efficiency below threshold (bank 1): rear oxygen sensor shows the cat is no longer cleaning the exhaust well.

Usual causes: worn or contaminated catalytic converter; exhaust leak before the rear sensor; faulty front air-fuel ratio sensor; misfire or oil burning damaging the cat

Volkswagen

Catalytic converter efficiency below limit on bank 1. Rule out leaks and the rear oxygen sensor before buying a cat.

Usual causes: exhaust or intake leak; worn catalytic converter; faulty oxygen sensor; oxygen sensor wiring

Volvo

Catalytic converter efficiency below threshold.

Usual causes: worn catalytic converter; rear O2 sensor; exhaust leak before the sensor

What the repair usually costs

Air-fuel ratio / front O2 sensor replace (upstream)NZ$270–NZ$750 · A$260–A$730
O2 sensor replace (downstream / after cat)NZ$180–NZ$480 · A$180–A$460
Catalytic converter replace, aftermarket direct-fitNZ$570–NZ$1,570 · A$540–A$1,490
Catalytic converter replace, genuineNZ$1,640–NZ$5,160 · A$1,470–A$4,590

Not sure what P0420 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.