P0420 Catalyst system efficiency below threshold (bank 1)
What P0420 means
The engine computer compares the oxygen sensor before the catalytic converter with the one after it. A working catalyst smooths out the signal; P0420 means the rear sensor is switching almost like the front one, so the catalyst is no longer storing oxygen effectively. The engine runs normally, but emissions are higher.
P0420 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 150–900 €
- Workshop time
- 1.5–5 h
- Repair difficulty
- Hard
- DIY possible
- No
- Check engine light
- Yes
- Affects emissions test
- Yes
- Limp mode possible
- No
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- No noticeable change in driving in most cases
- Rotten-egg smell from the exhaust
- Technical inspection failure for emissions
Common causes
- Worn-out or contaminated catalytic converter
- Faulty or slow downstream oxygen sensor
- Exhaust leak near the oxygen sensors
- Misfires or a rich mixture that overheated the catalyst
- Engine burning oil or coolant, poisoning the catalyst
Can I keep driving with P0420?
You can keep driving; this does not damage the engine. Have it checked soon, and fix any misfire codes first, because they can destroy a new catalyst.
What to check first
- Fix any misfire, fuel trim or oxygen sensor codes first
- Check for exhaust leaks between the manifold and the rear oxygen sensor
- Compare front and rear oxygen sensor signals in live data with a warm engine; a rear sensor that switches fast confirms a weak catalyst
- Replace the downstream sensor if it is slow or faulty, and the catalytic converter only if the tests confirm it
Related codes
- P0421 Warm-up catalyst efficiency below threshold (bank 1)
- P0411 Secondary air injection system incorrect flow detected
- P0430 Catalyst system efficiency below threshold (bank 2)
- P0410 Secondary air injection system A
- P0431 Warm-up catalyst efficiency below threshold (bank 2)
- P0406 EGR sensor A circuit high
- P0405 EGR sensor A circuit low
- P0404 EGR A control circuit range/performance
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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