Fault codes › Powertrain › Generic (SAE standard) › Auxiliary emission controls › Emission control systems
P0420 Catalyst system efficiency below threshold (bank 1)
Needs attention Have it looked at soon. The car is usually driveable with care in the meantime.
What does P0420 mean?
P0420 is a generic powertrain code, defined by the SAE standard, so it means the same thing on every make and model: catalyst system efficiency below threshold (bank 1). It belongs to the auxiliary emission controls group of codes.
A control module stores the code when something it watches stays outside what it expects for long enough. On its own the code says which system complained, not which part has failed — the causes below, most likely first, are where to start.
Is it safe to drive with P0420?
Needs attention. Driveable. Replacing the catalyst without fixing the underlying fuelling fault just repeats the failure.
Common causes of P0420
- Aged catalytic converter.
- Faulty rear oxygen sensor.
- Exhaust leak.
- Long-standing rich or lean running that poisoned the catalyst.
Symptoms
- Usually nothing to drive.
- Emissions test failure.
What to check first
- Compare front and rear oxygen sensor traces.
- Exhaust leaks.
- Fix any fuelling fault first.
Can I just clear P0420?
You can, and the light goes out — but a fault that is still there sets the code again, often within a drive or two. Clearing also erases the freeze frame (what the engine was doing when the code was stored) and resets the emissions readiness monitors, so an MOT, smog or inspection test will wait until they have run again. Fix the cause first, then clear it, then watch whether it returns.
Read and clear P0420 from your iPhone with OBDTrip
Plug a Bluetooth OBD-II adapter into the car and OBDTrip reads every stored, pending and permanent code, explains each one in plain English like this, shows what the engine was doing when it was set, and clears the check engine light once the fault is fixed. It keeps a history of when each code comes back, and records every drive on its own from then on.
Needs a Bluetooth Low Energy adapter such as the Vgate iCar Pro — which adapters work with iPhone. Reading and clearing codes is part of an OBDTrip subscription.
Related codes
- P0401 Exhaust gas recirculation flow insufficient
- P0402 Exhaust gas recirculation flow excessive
- P0430 Catalyst system efficiency below threshold (bank 2)
- P0440 Evaporative emission system
- P0442 Evaporative emission system small leak
- P0455 Evaporative emission system large leak
General guidance from OBDTrip's fault-code catalogue, not a diagnosis of your car. When a code points to brakes, steering, overheating or a flashing check engine light, have the car looked at before driving on.