P0421 fault code: what it means

P0421: Bank 1 warm-up (front) catalytic converter efficiency is below the limit.

Engine and gearbox (powertrain) · Costly if missed

Usual causes, most likely first

worn warm-up catalytic converter
slow rear O2 sensor
exhaust leak near the cat

P0421 by make

Mazda

Mazda uses P0421 for catalytic converter efficiency (petrol and diesel). Petrol: warm-up cat worn. Diesel: exhaust temps across the cat/DPF not rising as expected.

Usual causes: worn warm-up catalytic converter (petrol); loose or aged rear O2 sensor or front A/F sensor; exhaust leak before the sensors; exhaust temperature sensor No.2/No.3 fault (diesel)

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 P0421 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.