P0420 — Catalytic converter efficiency below threshold, Bank 1 — the downstream O2 sensor mirrors the upstream too closely, meaning the catalyst is no longer converting the exhaust properly
What it means
Common causes
- Catalyst wear — expected past 150–200K km, faster on gasoline engines running LPG
- Partial substrate damage — often after coolant intrusion or road impact
- Failed downstream O2 sensor (Bank 1 Sensor 2)
- Exhaust leak upstream or at the catalyst — cracked pipe, holed flex joint, blown gasket
- Long-term misfires or oil-fouled plugs — unburnt fuel destroys the ceramic substrate
- Aftermarket test pipe installed without proper O2 spacer/emulator — ECM sees no catalyst
- Failed upstream O2 sensor — bad mixture pushes catalyst to extremes
- Clogged EGR — excess exhaust gas accelerates catalyst wear
How to diagnose
- 01
Read all stored and pending codes with an OBD-II adapter — get the full picture, not just the one code.
- 02
Capture freeze-frame data (RPM, coolant temperature, engine load) at the exact moment the code triggered.
- 03
Inspect the named subsystem with our app's AI explanation — get vehicle-specific causes for your make, model and year.
Common patterns on specific vehicles
- Toyota Camry / RAV4 2.4/2.5
On Toyota 2.4/2.5, P0420 is a bad downstream O2 sensor in ~70% of cases (not the cat itself). Live data: if downstream O2 mirrors upstream, it's the sensor.
- VW / Audi 1.8T / 2.0T EA888
On EA888 engines past 75k miles, P0420 is often the cat itself. Cheaper to replace with the downstream O2 sensor at the same time.
- Honda CR-V / Civic 2.4
On Honda 2.4, P0420 is often caused by a leaky exhaust manifold gasket letting outside air in and skewing cat efficiency. Check gasket first.
Empirical mechanic observations. Your case may differ — use as a starting hypothesis, not a diagnosis.
Decode P0420 on your actual car
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