The computer compares the signal of the lambda probes before and after the catalyst. If the two show almost the same, the catalyst is not holding oxygen and is not purifying the gases sufficiently.
Low to medium. The car drives normally, but it won't pass MOT for emissions, and the cause could be another problem that damages the catalytic converter.
Gasoline engines. Catalytic converter, lambda probes, exhaust, engine (oil consumption, interruptions).
What does P0420 mean
A working catalyst "smoothes" the signal: the probe in front of it jumps quickly between lean and rich, and the one after it shows an almost flat line. When the catalyst gets old or dirty, the rear probe starts repeating the front one and the computer records P0420. On V- engines the same for the second side is P0430.
Symptoms
- "Check Engine" lamp, often no change in ride
- Smell of sulfur ("rotten eggs") from the exhaust
- If the catalytic converter is blocked - loss of power and high temperature under the car
- Failed emissions technical review
| Can it be driven?
Yes. But don't replace the catalytic converter blindly. If there are breaks in the engine or burning oil, the new catalytic converter will fail in the same way.
Common reasons
- Exhausted catalyst — normal wear and tear after long mileage.
- Faulty rear lambda probe — shows wrong and the catalyst looks bad (see P0136).
- Exhaust leak between the two probes or in front of them.
- Misfires (P0300) — the unburnt fuel melts the ceramic.
- The engine burns oil or antifreeze — deposits "clog" the catalyst.
- Continuous rich or lean mixture (P0172, P0171).
How we diagnose it
- We check for other codes — interruptions, mixture, probes. They are removed first.
- We watch in real time the signal of the two probes when the engine is warm.
- We check the exhaust for leaks and the rear probe.
- If the engine burns oil, we measure the compression and check the valve segments and seals.