P0420 Fault Code: What It Means and What To Do

What does P0420 mean?

P0420 means the engine management system has detected that the catalytic converter on bank 1 is not cleaning exhaust gases as efficiently as it should. The ECU monitors the catalytic converter by comparing the oxygen sensor readings before and after it. A healthy cat changes the composition of the exhaust gas significantly — the downstream sensor should read fairly flat compared to the upstream one. When they're reading similarly, the ECU concludes the converter isn't doing its job.

P0420 is extremely common on higher-mileage cars. It doesn't usually cause any noticeable change in how the car drives — you won't usually feel it — but it will result in MOT emissions failure and should be addressed before your next test.

Common causes

• Worn catalytic converter — the internal honeycomb structure degrades over time, particularly on high-mileage vehicles. Once the precious metal coating wears out, the cat can no longer break down harmful gases.
• Faulty downstream O2 sensor — a lazy or failed oxygen sensor after the cat can give misleading readings, triggering P0420 when the cat itself is still good. This is the first thing to check.
• Exhaust leak near the sensor — a small exhaust leak before or around the downstream sensor lets in fresh air, skewing the reading.
• Contaminated catalytic converter — oil burning or coolant entering the exhaust (from a head gasket leak) can coat and destroy the catalyst.
• Running rich or lean — persistent fuel mixture problems force the cat to work harder than it's designed to, accelerating its wear.

Typical UK repair costs

UK labour rates are typically £80–£120 per hour. Costs vary by vehicle and region.

What to do next

Don't assume the catalytic converter needs replacing until the oxygen sensor has been checked. A good mechanic will hook up a scanner with live data and watch how the upstream and downstream O2 sensors behave during a warm-up cycle. If the downstream sensor is sluggish or stuck at a fixed value, it's the sensor — a much cheaper fix than a new cat.

If the O2 sensor is fine and the cat is confirmed faulty, consider an aftermarket replacement — they typically meet MOT standards at a fraction of the OEM price on most common vehicles.