Worn catalytic converter. Faulty downstream O2 sensor. Exhaust leak. Engine misfire damaging the catalyst. Oil or coolant contamination.

Which cars are affected by P0420?

P0420 is most commonly reported on Toyota, Honda, Ford, Vauxhall, Nissan vehicles, but can appear on any make.

← All fault codes

P0420

Medium severity

Catalyst System Efficiency Below Threshold (Bank 1)

The catalytic converter on bank 1 is not operating efficiently enough. The downstream O2 sensor is showing readings too similar to the upstream sensor.

Get a full AI diagnosis for your vehicle

Enter your reg plate and fault code — Velocito.ai cross-references technical databases, MOT history, manufacturer recalls, and known fixes to pinpoint the exact cause.

Diagnose P0420 free3 free searches — no card required

Common causes of P0420

1Worn catalytic converter

2Faulty downstream O2 sensor

3Exhaust leak

+2 more causes

Get all causes ranked by likelihood for your specific vehicle — plus the cheapest confirming test for each.

Diagnose P0420 free

Commonly affected vehicles

ToyotaHondaFordVauxhallNissan

P0420 can appear on any vehicle — these are the makes where it is most frequently reported.

Full diagnosis report

Most likely cause for your vehicle

Based on your vehicle's make, model, year, and MOT history, the most probable cause is identified and ranked by likelihood.

Confirming diagnostic test

The cheapest test to confirm the root cause before replacing any parts — saving time and money on unnecessary work.

Recalls & technical bulletins

Manufacturer recalls and TSBs checked automatically for your specific vehicle, so nothing gets missed.

Unlock the full report

Enter your reg plate to get causes ranked for your specific vehicle, confirming tests, recall checks, and known fixes.

Try 3 free searches

Stop guessing. Start diagnosing.

Enter your reg plate and P0420 — get a full diagnostic report in seconds.

Try 3 free searches

Related fault codes

P0171Medium

System Too Lean (Bank 1)

P0300High

Random/Multiple Cylinder Misfire Detected

P0172Medium

System Too Rich (Bank 1)

P0401Medium

Exhaust Gas Recirculation (EGR) Flow Insufficient