P0420 Catalyst System Efficiency Below Threshold Bank 1

What does fault code P0420 mean?

P0420 indicates your catalytic converter is failing. UK repair costs, causes and fixes for OBD2 fault code P0420. When this fault is detected, the ECU (Engine Control Unit) stores the code and illuminates the check engine light (also called the engine management light or malfunction indicator lamp) on your dashboard.

All cars sold in the UK since 1996 use the standardised OBD-II diagnostic system, meaning fault code P0420 can affect any manufacturer — including Ford, Vauxhall, BMW, Volkswagen, Toyota, Mercedes-Benz, Audi, Renault, Peugeot, Nissan, Honda, Hyundai, Kia, and all others. The code is read using an OBD-II scanner plugged into the 16-pin diagnostic port, usually located under the dashboard on the driver's side.

Symptoms of fault code P0420

These are the most common symptoms UK drivers report when this code is active:

• Check engine light illuminated
• Reduced fuel economy (10–20% worse than normal)
• Sulphur or rotten egg smell from the exhaust
• Failed MOT emissions test
• Rattling noise from under the car in severe cases

Note that some vehicles show no drivability symptoms at all — the check engine light may be the only sign. This is why scanning the fault code is essential rather than guessing at the cause.

What causes P0420?

Listed from most to least common based on UK garage data:

1. Failing or failed catalytic converter (most common)
2. Faulty downstream oxygen sensor (O2 sensor Bank 1 Sensor 2)
3. Exhaust leak before or after the catalytic converter
4. Engine running rich — excess unburnt fuel damages the catalyst
5. Misfires sending raw fuel into the exhaust system

It is important to diagnose the root cause before replacing parts. Many unnecessary and expensive repairs happen because the most expensive component is replaced first without proper testing. Always work through the diagnostic steps systematically.

How to fix P0420 — step by step

Work through these steps in order to avoid unnecessary parts replacement:

1. Check for related codes first — P0300–P0308 misfires or P0171/P0172 fuel trim codes must be fixed before the cat
2. Test both oxygen sensors with live data — a lazy or dead downstream sensor often mimics a failed cat
3. Inspect the exhaust system visually for cracks, rust holes or loose joints
4. If sensors test fine, the catalytic converter itself needs replacing — expect £250–£800 for parts plus 1–2 hours labour

After any repair, clear the fault code with an OBD-II scanner and carry out a test drive. If the code returns, continue to the next diagnostic step. For a personalised repair guide specific to your vehicle's make, model, year and engine, use our AI Diagnostic Report below.

UK vehicles commonly affected by P0420

These models are frequently seen with this fault code in UK garages:

• Ford Focus (2005–2015)
• Vauxhall Astra (2004–2014)
• Volkswagen Golf (Mk5, Mk6)
• Toyota Corolla/Auris
• BMW 3 Series (E46, E90)
• Nissan Qashqai (2007–2017)

This code can appear on any OBD-II vehicle (all UK cars since 1996) but is more common on the models listed above. If your vehicle is not listed, it does not mean the code cannot affect it — it simply means these models are statistically more frequent in UK repair data.

DIY difficulty and cost guide

Difficulty: Medium — oxygen sensors are DIY-friendly (£50–£120 parts). Catalytic converter replacement usually requires a lift and specialist tools.

UK repair cost estimate: £150–£800 including parts and labour at average UK workshop rates (inc. VAT). Independent garages are typically 30–50% cheaper than main dealers. Always get at least two quotes.

Prevention: Keep up with engine servicing, fix misfires immediately, avoid cheap unbranded fuel, and do longer motorway runs to keep the catalyst hot and self-cleaning.

P0420 and the UK MOT test

Fault code P0420 is likely to cause an MOT failure. Under current DVSA testing standards, an illuminated malfunction indicator lamp (MIL / check engine light) is an automatic MOT failure, regardless of whether the vehicle is otherwise roadworthy. This means you must resolve this fault before your MOT test.

Even if the light is not currently on, if the fault is intermittent it could reappear during the test. Use our AI Diagnostic Report to understand the MOT risk specific to your vehicle and get advice on whether it is safe to proceed to your test.

P0420 guides by vehicle make

Select your vehicle for a specific diagnosis guide with UK repair costs:

Frequently asked questions about P0420

Related OBD2 fault codes

These fault codes are commonly found alongside P0420 or share similar causes: