P0420: Catalytic converter efficiency below threshold (bank 1)
The bank 1 catalytic converter is no longer cleaning the exhaust properly. Usually it is worn, but an oxygen sensor or exhaust leak can be to blame.
| Code | P0420 |
|---|---|
| System | Engine and transmission |
| Group | P04xx, Emissions control |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What P0420 means
The catalytic converter turns the most harmful exhaust gases into less harmful ones. The ECU monitors it with two oxygen sensors: if the one after it switches almost like the one before, it concludes the converter is not doing its job. Bank 1 is the side of the engine with cylinder 1; inline engines only have bank 1.
Most common causes
- Exhaust leak near the oxygen sensors (gasket or pipe)
- Slow or faulty downstream oxygen sensor (sensor 2)
- Corroded oxygen sensor connector
- Worn out or damaged catalytic converter
- Misfires or oil burning that have damaged the converter
Symptoms
- Engine warning light on
- Failing an emissions test
- Rotten egg smell from the exhaust
- Sometimes loss of power if the converter is blocked
Can you drive with P0420?
With care You can drive, but if there are misfires or the converter becomes blocked, it can get worse. Take it to a garage soon and before the next inspection.
Open the bonnetthe in-depth explanation
How to check it
With the engine warm, watch the oxygen sensor voltages on your reader: the upstream one (sensor 1) should switch quickly and the downstream one (sensor 2) should stay fairly steady. If both switch the same way, the converter is worn.
How to read and clear P0420 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0420 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P041C The exhaust gas recirculation temperature sensor B gives a lower signal than
- P041D The exhaust gas recirculation temperature sensor B gives a higher signal than
- P041E The exhaust gas recirculation temperature sensor B fails now and then
- P041F The secondary air injection system switching valve A gives a lower signal than
- P0421 Warm up catalyst efficiency below threshold (bank 1)
- P0422 The main catalytic converter on bank 1 works below the required efficiency
- P0423 The heated catalyst on bank 1 is working below the minimum efficiency threshold
- P0424 The heated catalyst temperature on bank 1 is below its minimum threshold
- P0400 Exhaust gas recirculation (EGR) flow malfunction
- P0401 Exhaust gas recirculation (EGR) flow insufficient
- P0402 Exhaust gas recirculation (EGR) flow excessive
- P0403 EGR control circuit malfunction
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: Misfires, catalyst and lambda sensors.
What does code P0420 mean?
The catalytic converter turns the most harmful exhaust gases into less harmful ones. The ECU monitors it with two oxygen sensors: if the one after it switches almost like the one before, it concludes the converter is not doing its job. Bank 1 is the side of the engine with cylinder 1; inline engines only have bank 1.
Can you drive with P0420?
You can drive, but if there are misfires or the converter becomes blocked, it can get worse. Take it to a garage soon and before the next inspection.
How do you clear P0420?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0420 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
