A P0420 fault code on your OBD2 scanner points to a catalytic converter problem — and the first question most UK drivers ask is how much it's going to cost. The honest answer is: it varies enormously, and there's a good chance you don't even need a new catalytic converter. This guide covers everything you need to know about UK catalytic converter replacement costs in 2026, how to avoid being overcharged, and what to check before authorising any work.
The catalytic converter is part of your car's exhaust system. It uses precious metals — platinum, palladium and rhodium — to convert harmful exhaust gases (carbon monoxide, hydrocarbons and nitrogen oxides) into less harmful carbon dioxide, water and nitrogen before they exit the tailpipe. When the cat degrades and can no longer do this efficiently, the car fails the MOT emissions test and stores fault code P0420 (bank 1) or P0430 (bank 2).
| Car make/model | Parts cost (aftermarket) | Total fitted (indie garage) | Total fitted (main dealer) |
|---|---|---|---|
| Ford Fiesta / Focus | £80–£180 | £200–£380 | £350–£600 |
| Vauxhall Astra / Corsa | £80–£200 | £200–£400 | £350–£650 |
| Volkswagen Golf / Polo | £120–£280 | £280–£500 | £450–£800 |
| BMW 3 Series | £200–£500 | £400–£800 | £700–£1,400 |
| Audi A3 / A4 | £180–£450 | £380–£750 | £650–£1,300 |
| Toyota Corolla / Yaris | £100–£250 | £250–£480 | £400–£750 |
| Honda Civic | £100–£280 | £250–£500 | £400–£800 |
| Nissan Qashqai | £120–£300 | £280–£550 | £450–£850 |
| Mercedes C-Class | £250–£600 | £500–£1,000 | £900–£1,800 |
| Land Rover / Range Rover | £300–£700 | £600–£1,200 | £1,100–£2,200 |
Prices are estimates based on 2026 UK market data. Labour rates vary by region — London and South East typically 20–30% higher than the Midlands and North.
The most important thing to know about P0420 is that the code means the catalyst system is not working efficiently — it does not definitively identify the catalytic converter as the failed component. Before spending £300–£800 on a new cat, always check:
The ECU determines catalytic converter efficiency by comparing the signal from the upstream (before the cat) and downstream (after the cat) oxygen sensors. If the upstream sensor is lazy, slow or failing, it can give a false reading that triggers P0420 even when the catalytic converter itself is fine. An upstream O2 sensor costs £30–£80 for the part and £50–£120 for fitting — a fraction of a new catalytic converter. This should always be investigated before replacing the cat.
A small exhaust leak upstream of the catalytic converter allows extra oxygen into the exhaust stream, which confuses the oxygen sensor readings and can trigger P0420. Inspect the exhaust manifold, manifold gasket, and downpipe for cracks or leaks before assuming the cat is at fault.
If the engine has been running with active misfires (P0300–P0306), running rich, or has been consuming oil, the catalytic converter may have been damaged by these upstream faults. Replacing the cat without fixing the root cause will simply destroy the new one within months. Always fix any misfire, fuel trim or oil consumption issues before replacing the catalytic converter.
On some vehicles — particularly certain VW Group and Ford models — P0420 can be triggered by an overly sensitive ECU threshold that has since been revised. A software update from a main dealer or independent with the correct equipment can sometimes resolve P0420 without any parts replacement.
| Type | Cost (parts only) | Pros | Cons |
|---|---|---|---|
| OEM (manufacturer) | £300–£1,200+ | Perfect fit, guaranteed efficiency, longest life | Very expensive, often only from dealers |
| Quality aftermarket (Klarius, BM Cats, Bosal) | £80–£400 | Meets Euro emissions standards, passes MOT, good lifespan | May not last as long as OEM on high-mileage cars |
| Budget/unbranded | £40–£100 | Cheap upfront | May not pass MOT emissions test, short lifespan, false economy |
Labour rates vary significantly across the UK. Catalytic converter replacement typically takes 1–2 hours of labour:
| Region | Typical hourly rate | Cat replacement labour |
|---|---|---|
| London | £90–£130/hr | £130–£260 |
| South East | £75–£110/hr | £110–£220 |
| Midlands | £60–£85/hr | £90–£170 |
| North of England | £55–£80/hr | £80–£160 |
| Scotland / Wales | £55–£80/hr | £80–£160 |
Catalytic converter replacement is one of the most common areas where UK drivers are overcharged. Here's how to protect yourself:
If the catalytic converter is genuinely the cause of the P0420 fault, a quality replacement will resolve the code and allow the car to pass the MOT emissions test. However, after the repair:
Find out whether your P0420 is likely a sensor or the catalytic converter — based on your exact year, make, model and engine. Avoid an unnecessary £400+ repair.
Get My AI Diagnostic Report — £1.59 →Disclaimer: AI-Diagnostics-Pro provides information for educational purposes only. Repair cost estimates are based on UK market data and may vary by region, vehicle and garage. Always consult a qualified mechanic before carrying out vehicle repairs.