OBD-II diagnostic trouble code
P0420Safe to drive
Catalyst System Efficiency Below Threshold (Bank 1)
The engine computer compares the oxygen readings before and after the catalytic converter on bank 1. If the readings look too similar, it means the catalyst isn't cleaning the exhaust as effectively as it should anymore, so the car flags it — even though the engine itself may run perfectly fine.
Symptoms
- Often no noticeable driving symptoms at all — this is frequently a "check engine light only" code
- Possible failed emissions/MOT/TÜV test
- A faint sulfur or rotten-egg smell from the exhaust in some cases
- Slightly reduced fuel economy if the underlying cause is an engine issue feeding the catalyst too much unburned fuel
Likely causes
- Aging, naturally worn-out catalytic converter — very common on cars over 8-10 years or 150,000+ km
- An upstream problem (misfire, oil consumption, or rich fuel mixture) that's contaminating and damaging the catalyst
- A failing rear (post-catalyst) oxygen sensor giving a false reading rather than the catalyst itself being bad
- Exhaust leak near the sensors, letting outside air skew the readings
How to diagnose it
- Check for other active or pending codes first (misfires, fuel trim codes) — fixing those often resolves the catalyst code without replacing anything
- Compare live upstream vs. downstream oxygen sensor data with a scan tool — a genuinely worn catalyst shows near-identical waveforms front and back
- Inspect the exhaust for physical damage or leaks near the catalyst and sensors
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Code names are compiled from open/standardized SAE and ISO references. Explanations, symptoms, causes and fixes are original. Covers generic (P0/C0/B0/U0) codes only — manufacturer-specific codes are planned for a future update.
Frequently asked questions
Can I drive with P0420?
In most cases, yes — P0420 is generally not an urgent safety issue and the car is usually safe to drive for a while. Still, get it checked soon, since the underlying cause can affect fuel economy or emissions the longer it's left.
How much does it cost to fix P0420?
Typical repair costs for {code} range from roughly 0 to 1800 EUR, depending on the specific cause and your car — see the fixes listed above for a breakdown.
What causes P0420?
Aging, naturally worn-out catalytic converter — very common on cars over 8-10 years or 150,000+ km An upstream problem (misfire, oil consumption, or rich fuel mixture) that's contaminating and damaging the catalyst A failing rear (post-catalyst) oxygen sensor giving a false reading rather than the catalyst itself being bad
Reviewed by Artyom SemenovAutomotive Editor · Fact-checked by Yauheni Kapliarchuk, Editor-in-Chief