P0420: Catalyst System Efficiency Below Threshold Bank 1

The engine module checks whether the catalyst of bank 1 still converts enough. On petrol engines it estimates the oxygen storage capacity by comparing the sensors before and after the catalyst – a good catalyst smooths the rear signal. GM uses a test on overrun (ratio below 0.35 sets the code on the first trip), Ford an index ratio above 0.75 or fuel integration after fuel cut-off. On diesels P0420 concerns the oxidation catalyst, checked by its heat release during filter regeneration (GM Duramax LML, Ford below 45 % of the expected heat). Ford names oil-borne poisoning as the most common cause.

Low severity. Can I keep driving?Yes, if the car is running normally, although it pollutes more and with the light on you may have problems at an emissions inspection. Stop and get it checked right away if the light flashes, you notice a loss of power, or you smell burning, because the catalytic converter may be clogged or overheating.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

Check engine light on, otherwise often no change in driving
Emission test failed (higher HC, CO, NOx)
Rattling or loss of power if the catalyst has broken up

Most likely causes

Catalyst aged or poisoned (oil consumption, silicone, coolant)
Catalyst overheated by misfire or rich running
Rear oxygen sensor sluggish, dirty or unsuitable
Exhaust leak ahead of or at the rear sensor
Diesel: oxidation catalyst sooted or poisoned, post-injection faulty

What to check, in order

Start with the simplest. You can tick off each step.

First resolve any misfire or fuel mixture codes
Look for exhaust leaks between the engine and the second sensor
Use live data to compare the signals from both oxygen sensors
Check for a blockage by measuring exhaust back pressure
Replace the catalytic converter only at the end of the diagnosis

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0420 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
20: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0420?

Yes, if the car is running normally, although it pollutes more and with the light on you may have problems at an emissions inspection. Stop and get it checked right away if the light flashes, you notice a loss of power, or you smell burning, because the catalytic converter may be clogged or overheating.

What causes code P0420?

The most common causes are: catalyst aged or poisoned (oil consumption, silicone, coolant); catalyst overheated by misfire or rich running; rear oxygen sensor sluggish, dirty or unsuitable.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.