P0420: Catalyst system efficiency below threshold (bank 1)
The catalytic converter is not cleaning the exhaust as well as it should, judged by the oxygen sensors.
Updated
What P0420 means
The catalytic converter cleans the exhaust before it leaves the car. To check that it still works, the engine computer compares two oxygen sensors: one in front of the converter and one behind it.
A working converter evens out the exhaust, so the sensor behind it gives a steadier signal than the one in front. When the rear signal starts to follow the front one, the converter isn’t doing enough, and the engine computer stores P0420.
Bank 1 is the side of the engine with cylinder 1. An engine with its cylinders in one row has only bank 1.
P0430: the same fault on bank 2
V and flat (boxer) engines have two banks of cylinders, and many have a converter and oxygen sensors for each. P0430 is the same test failing on bank 2, the side without cylinder 1. The meaning, causes and advice are the same: check the sensors and exhaust on that side.
P0430: Catalyst system efficiency below threshold (bank 2)
Can I keep driving?
Likely OK to drive
Likely OK to drive for now. Have it fixed before the next emissions inspection (APK, MOT, TÜV, contrôle technique).
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Look at the other codes first. Misfire codes (P0300 to P0312) are more urgent: unburnt fuel can overheat the catalytic converter. If the check engine light flashes, stop when it’s safe to do so.
At an emissions inspection that reads OBD, a stored P0420 means the OBD check isn’t passed. In the Dutch APK, for example, cars from 2006 that run on petrol then get the exhaust measured instead.
Symptoms
- The check engine light is on. Often that’s the only sign: the car drives as before.
- On some cars, a rotten-egg (sulphur) smell from the exhaust.
- Other codes stored at the same time, such as misfire, fuel mixture or oxygen sensor codes. These often point to the real cause.
Common causes
- Aged or damaged catalytic converter. Converters wear out with age and mileage, and can be damaged by heat, or by oil or coolant burning in the engine.
- Faulty rear oxygen sensor. A slow or failing sensor behind the converter can make a good converter look bad.
- Exhaust leak before the sensor. Outside air drawn in through a leak upsets the rear sensor’s reading.
- Earlier misfires that damaged the catalyst. Unburnt fuel burns inside the converter and overheats it.
Typical fixes
- Read all fault codes, and fix any misfire or fuel mixture fault first. Clearing P0420 while those remain only brings it back.
- Check the exhaust for leaks between the engine and the rear oxygen sensor, and repair them.
- Have the rear oxygen sensor and its wiring tested, and replace the sensor if it’s faulty.
- Replace the catalytic converter only when the other causes are ruled out, with one approved for your car.
- After the repair, clear the codes and drive normally for a few days. The Catalyst self-test under emissions readiness shows when the car has re-tested the converter.
Questions
Can I drive with a P0420 code?
Usually, for now. On its own it’s likely OK to drive, but have it fixed before the next emissions inspection. If misfire codes are stored too, avoid hard driving and have it checked soon, and if the check engine light flashes, stop when it’s safe to do so.
Does P0420 always mean a bad catalytic converter?
No. A faulty rear oxygen sensor or an exhaust leak in front of it can set the same code. Rule those out before replacing the converter.
Will clearing P0420 turn the light off for good?
No. Clearing doesn’t repair anything: if the cause is still there, the light comes back. On cars that keep permanent codes, P0420 stays listed until the car re-tests the repair.
What is the difference between P0420 and P0430?
They are the same fault on different sides of the engine. P0420 is bank 1, the side with cylinder 1. P0430 is bank 2, which only engines with two rows of cylinders have, such as V and flat engines.