P0420
Catalyst System Efficiency Below Threshold Bank 1.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 4
- AreaAuxiliary emission controls
- 20
- FaultThe specific fault in that area
What it means
P0420 means the engine control unit has judged that the catalytic converter on bank 1 is not cleaning the exhaust as well as it should. It makes this judgement by comparing the signal from the oxygen sensor before the catalyst with the one after it. A healthy catalyst smooths out the downstream signal, whereas a tired one lets it swing more like the upstream sensor.
Symptoms
- Engine warning light on
- Usually no change in how the car drives
- Slightly higher fuel consumption in some cases
- Failed emissions test
- Sulphur or rotten egg smell from the exhaust in some cases
- Reduced power if the catalyst is partly blocked
Possible causes
- Catalytic converter on bank 1 worn, contaminated or damagedlikely
- Downstream (post-catalyst) oxygen sensor slow, ageing or faultylikely
- Exhaust leak before or near the oxygen sensors letting in false air readingspossible
- Engine misfire or poor combustion overheating or contaminating the catalystpossible
- Upstream oxygen sensor or air-fuel sensor fault, or fuel trim problem causing a rich or lean mixturepossible
- Damaged wiring or poor connector at the oxygen sensorsrare
- Engine oil or coolant burning in the cylinders and coating the catalystrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, and look for misfire, fuel trim or oxygen sensor codes that need fixing first.
- Check for any other recorded faults or service bulletins relating to the model and engine, as catalyst monitor behaviour varies between vehicles.
- Inspect the exhaust from the manifold to the rear of the catalyst for leaks, cracks, blown gaskets and damage.
- Inspect the oxygen sensor wiring and connectors for damage, heat marks, oil contamination or corrosion.
- Check short and long term fuel trims at idle and at 2,500 rpm to spot lean or rich running.
- Watch live upstream and downstream sensor readings: the upstream should switch or respond quickly, while the downstream should be fairly steady on a healthy catalyst.
- Check for signs of oil or coolant consumption, such as blue smoke, white deposits or a sweet smell.
- If sensors and engine are sound, check the catalyst condition by temperature difference or backpressure, following the procedure for that model.
How to fix
- Repair any exhaust leaks found upstream of or around the catalyst.
- Fix misfires or fuel trim faults, such as ignition parts, injectors or air leaks, before judging the catalyst.
- Replace a faulty downstream or upstream oxygen sensor once testing confirms it is at fault.
- Repair damaged wiring or connectors at the oxygen sensors.
- Replace the bank 1 catalytic converter if testing shows it has failed, and fix the cause of its damage first.
- Fit an updated engine control unit calibration only where the manufacturer lists it as the fix for this model.
Can I keep driving?
The car can normally be driven while you book it in, as this code alone rarely affects drivability. If there is also a misfire, flashing warning light or loss of power, stop driving and have it checked, because unburnt fuel can quickly damage the catalyst.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.