Code P0420: catalytic converter at fault, do you really need a new one?

What code P0420 means

Your engine has two O2 sensors (lambda sensors): one before the catalytic converter (upstream) and one after it (downstream). The control unit compares their signals. A healthy catalytic converter “smooths out” the exhaust gases: the upstream sensor switches quickly while the downstream sensor stays almost flat. When the downstream sensor starts following the upstream one, the control unit concludes that the catalytic converter is no longer doing its job and stores P0420 (bank 1). On a V6 or V8, the same fault on the other row of cylinders gives P0430.

The catch: the control unit does not see the catalytic converter. It infers. And several other faults produce exactly the same electrical symptom.

Is it serious? Can I keep driving?

The engine is generally not at risk in the short term. However:

• a lit check engine light is a major defect at the roadworthiness test (contrôle technique in France, similar to the UK MOT) → re-test;
• if the real cause is a misfire or a mixture that is too rich, the catalytic converter will end up truly destroyed (it overheats and melts).

The most common causes

1. Worn downstream O2 sensor: it responds poorly, so the control unit thinks the catalytic converter is dead. A very common cause on cars with more than 150,000 km.
2. Exhaust leak between the engine and the downstream sensor (manifold gasket, flex pipe, weld): air gets in and skews the reading.
3. Misfires or a leaking injector: unburnt fuel reaches the catalytic converter and makes it overheat. Check whether there are also P030x codes.
4. Oil consumption (piston rings, valve stem seals): oil fouls the catalytic converter.
5. Catalytic converter genuinely worn or broken (rattling at the rear, loss of power).
6. Low-end replacement catalytic converter fitted recently: some do not pass the control unit's test.

What to check first

1. Note down all the codes (“Read fault codes”, including the pending codes), not just P0420. A P0300, P0171 or P0135 at the same time completely changes the order of the checks. You can look up each code in the fault code dictionary.
2. Display both O2 sensors in real time (the “Sensors” screen of DiagAuto AI, when the car provides their voltage), engine warm, at idle and then at 2,500 rpm:
   • upstream switching between ~0.1 and 0.9 V and downstream almost flat around 0.6–0.7 V → the catalytic converter is probably fine, suspect the downstream sensor or a leak;
   • downstream switching like the upstream one → the catalytic converter is probably dead.
3. Listen to the exhaust with the engine cold: a hiss or a metallic ticking near the manifold points to a leak.
4. Do a 20–30 minute motorway drive and then read the codes again: sometimes the code comes from light fouling that burns off when hot.

Should you clear the code?

You can clear it with the app, after confirming (a standard OBD2 command: DiagAuto AI never reprograms anything). If it comes back after a few dozen kilometres, the fault is real. Be careful: clearing the code just before the inspection solves nothing. The emissions checks (the “readiness monitors”) go back to “not ready”, the light comes back on if the fault is still there, and on some recent vehicles the inspector may also query the diagnostic socket.

Indicative cost (France, 2026)

Read also: Check engine light on but the car drives fine and Roadworthiness test: check your codes beforehand.