P0420 — Catalytic converter not working efficiently
The engine's emissions system is detecting that the catalytic converter isn't cleaning exhaust gases as well as it should. This often happens because the converter is worn out or damaged, or because sensors are giving incorrect readings.
Common causes
The cause is almost always one of a few things. Here they are — P0420 rarely comes from anything outside this list:
- Worn or damaged catalytic converter
- Faulty oxygen sensor before or after converter
- Exhaust leaks near sensors
- Engine running rich or misfiring
- Contaminated catalytic converter from oil or coolant
Where to start
In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:
- Check for and repair any exhaust leaks
- Test and replace faulty oxygen sensors
- Inspect engine for misfires or fuel issues
- Clean or replace catalytic converter if needed
That is what holds for P0420 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.
Why TorqFix for your car?
Above is what the code means and why it appears. TorqFix goes further — it gives you specific procedures, torque values, wiring diagrams and pinouts for your exact engine, not generic steps for every car. Real workshop data (127,000+ diagrams, 10,000+ procedures), interpreted by AI in your language.
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Code breakdown
🔧 Related repair procedures
- VW: Removing and installing catalytic converter
- AUDI: Removing and installing front exhaust pipe (right-side) with catalytic converter - vehicles with multitronic gearbox
- AUDI: Checking Lambda probe and Lambda control before catalytic converter
- AUDI: Removing and installing front exhaust pipe (left-side) with starter catalytic converter and main catalytic converter (vehicles with automatic gearbox)
- AUDI: Removing and installing starter catalytic converter
- AUDI: Removing and installing front exhaust pipe (left-side) with starter catalytic converter and main catalytic converter (vehicles with manual gearbox)
- AUDI: Removing and installing front exhaust pipe (right-side) with starter catalytic converter and main catalytic converter (vehicles with automatic gearbox)
- VW: Removing and installing catalytic converters
- VW: Removing and installing starter catalytic converter
- VW: Checking Lambda probe and Lambda control before catalytic converter
Who makes these pages, and how
TorqFix was built by a mechanic — after a 2,700 km drive with five roadside repairs that no diagnostic tool could explain.
The figures on this page come from manufacturer workshop documentation and are shown as they are. The page is assembled automatically from that data — it is not written by an AI. The AI works in the chat, where it answers for your specific car. It writes nothing here.
The same code can mean different things on different makes. Match the meaning to your make before acting on it.
Seen a figure that does not match your manual? Write to us — we correct it and note when. support@torqfix.com
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