P0420
PowertrainCatalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter on bank 1 isn't working efficiently. The ECU compares signals from the front oxygen sensor (before the cat) and the rear sensor (after the cat); when the rear signal starts mirroring the front instead of staying stable, the ECU concludes the cat isn't doing its job and sets P0420. In practice, this nearly always means an aged cat on a higher-mileage car, and the decision becomes whether to fix it or sell the car as-is.
ⓘ Information only. This page provides general educational information about fault code P0420. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0420 mean?
P0420 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Catalyst System Efficiency Below Threshold (Bank 1).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, often the only obvious symptom
- • Sometimes a faint sulphur or 'rotten egg' smell from the exhaust, particularly under load
- • Slight reduction in fuel economy, often only noticeable on long motorway runs
- • On a few vehicles, a noticeable rattling sound from the catalytic converter housing as the substrate breaks up internally
- • Slightly worse acceleration if the cat is partially blocked
- • On older cars: blue smoke from the exhaust if oil consumption has been the underlying cause of cat damage
Possible causes
Causes commonly associated with P0420, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Catalyst aged or contaminated, the most common cause on cars over 80,000 miles. The substrate gradually loses its catalytic activity over time and heat cycles
- 2. Faulty downstream (post-cat) oxygen sensor giving inaccurate readings, the cat is fine but the sensor is reporting wrong data
- 3. Faulty upstream (pre-cat) oxygen sensor causing incorrect fuel mixture that damages the cat
- 4. Engine misfire damaging the catalyst over time, hot unburned fuel hitting the substrate destroys it. This is the silent killer of cats — fix misfires fast
- 5. Engine running rich for a sustained period (failed coolant temp sensor, leaking injector, fuel pressure too high)
- 6. Exhaust leak ahead of the rear oxygen sensor letting outside air into the reading
- 7. Aftermarket performance parts (decat, sports cat) — the code is cosmetic in that case but still fails MOT
- 8. On rare occasions: coolant leak into the cat (head gasket failure) coating the substrate and disabling it
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Look here first: read live data from both oxygen sensors. A healthy cat shows a stable rear sensor signal (steady around 0.6-0.8V) compared to the rapidly oscillating front sensor. If the rear is also oscillating, the cat is genuinely degraded
- 2. Check for any other stored codes — misfires (P0300 series) or fuel trim codes (P0171, P0174) often precede a P0420 and need fixing first. Replacing the cat without fixing what's killing it just damages the new cat
- 3. Inspect the exhaust visually for cracks, leaks, or impact damage; an exhaust leak before the rear sensor causes false P0420 codes
- 4. Check fuel trim values to confirm the engine isn't running rich, long-term fuel trim well below zero (-15% or worse) damages the cat
- 5. Test the rear oxygen sensor by swapping it with a known-good one if available. Some P0420s are just lazy rear sensors at high mileage
- 6. If sensors and fuel trim are healthy, the catalyst itself is the likely culprit. At this point, the real question is whether to fix it or move on
Common questions about P0420
Is it worth fixing a P0420 on an older car, or should I just sell? +
Genuinely depends on the rest of the car's condition and value. An aftermarket cat plus rear oxygen sensor on a typical UK family car costs £400-£800 fitted at an independent garage. If your car is worth £1,500-£3,000 and otherwise reliable, that's worth doing. If the car is already on its last legs (other faults, MOT due, body issues), selling it as-is to a salvage buyer or to car.co.uk for a fair instant offer often makes more sense than spending £600 on a cat for a car you'd sell at auction for £400.
Will P0420 fail my MOT? +
The code itself doesn't directly cause MOT failure, but if the engine warning light is illuminated at the time of the test, that's an automatic fail under post-2018 rules. Drive a few cycles to clear the light if you've fixed the underlying cause. If the cat is genuinely failed, the emissions test will also typically fail, the cat is what cleans up the exhaust gases that the test measures.
Can I just fit a cheap aftermarket cat? +
Quality aftermarket cats from reputable UK brands work as well as OE for daily drivers and cost a third of the price. Avoid the cheapest no-name parts — they sometimes don't get certified for emissions standards, and on newer cars (post-2018) the ECU is sensitive to non-OEM substrates and may set P0420 again with a cheap replacement. Look for VCA-approved aftermarket parts. Don't trust eBay specials.
Will fuel additives or 'cat cleaners' fix this? +
Honestly, rarely. Cat-cleaner additives can temporarily improve a marginal cat but won't repair an aged or damaged one. If the substrate has broken down or melted, no additive will rebuild it. They're a delay tactic at best, not a real fix. Don't spend £30 on miracle additives, that's a sixth of the cost of a quality aftermarket rear oxygen sensor which actually fixes a meaningful share of P0420s.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →