OBD Code P0420: Catalyst System Efficiency Below Threshold — Causes & Fix

What does code P0420 mean?

Code P0420 (Catalyst System Efficiency Below Threshold — Bank 1) means the catalytic converter on bank 1 is no longer converting exhaust gases efficiently. The ECU compares the upstream and downstream oxygen sensor signals: when they're too similar, the cat isn't doing its job.

This is one of the most common check engine codes in the UK. It typically appears after 100,000–120,000 miles, though premature failures occur with misfires, rich running, or cheap aftermarket cats.

Symptoms of P0420

• Check engine light (solid)
• Smell of rotten eggs from the exhaust
• Slightly reduced fuel economy
• Possible failed MOT emissions test

Common causes

1. Worn catalytic converter (60% of cases)

After high mileage, the precious metal coating inside the cat degrades. The most cost-effective first step is a catalytic converter cleaner poured into the fuel tank.

👉 Catalytic converter cleaner on Amazon UK — £10-15, works within one tank if the cat isn't completely dead

2. Faulty downstream oxygen sensor (25%)

A slow or biased downstream lambda sensor gives a false P0420. Test: with a live data scanner, watch the downstream O2 signal — it should be relatively flat when the engine is warm. If it oscillates like the upstream sensor, the sensor is bad.

👉 Lambda sensor universal on Amazon UK — £20-60, Bosch recommended

3. Engine misfire (P0300/P030x)

Unburnt fuel destroys catalytic converters. Fix any misfire codes before replacing the cat.

4. Oil burning or coolant leak

Burning oil or coolant poisons the cat. Check for blue or white smoke first.

How to diagnose P0420 yourself

1. Get an OBD2 scanner with live data capability
2. Watch both upstream and downstream O2 sensor readings
3. If downstream signal fluctuates rapidly like upstream → sensor HS
4. If downstream signal is flat and near stoich → cat HS
5. Try a bottle of cat cleaner additive for 2 tanks before condemning the cat

👉 OBD2 Bluetooth scanner on Amazon UK — from £20

Repair cost

Can I drive with P0420?

Yes, short term. The cat has already degraded — there's no additional risk from driving. However: if your MOT is due, the light will likely cause a failure. Fix before the test.

FAQ

Will P0420 cause an MOT failure? Since 2018 the MOT checks the OBD port for fault codes. A lit check engine light = automatic failure.

I cleared the code and it came back in 50 miles — what now? The cat or O2 sensor is genuinely faulty. Try the cleaner additive first — if the code returns within 2 tanks, move to sensor or cat replacement.