P0420 Repair Cost UK — What Drivers Usually End Up Paying

Published 1 Jun 2026 • Catalyst faults • UK repair costs

P0420 can be a relatively cheap fix or a four-figure mistake, depending on what is actually wrong. The code tells you the catalyst-efficiency test failed. It does not tell you whether the root cause is an exhaust leak, a lazy oxygen sensor, a previous misfire, or the catalytic converter itself.

Quick answer

P0420 repair cost in the UK can range from relatively cheap leak or sensor work to a costly catalytic-converter replacement, so the code alone does not justify the biggest quote. The best repair path is to rule out leaks, sensor issues, and related misfire or fueling faults before treating the converter as proven bad.

Methodology and trust

This guide uses common UK repair-cost ranges and standard catalyst-efficiency diagnosis to explain why P0420 quotes differ so widely. It is intended to give pricing context and better questions to ask, not replace proper exhaust and live-data testing. For how we explain diagnostics and review tools, see our editorial approach, customer reviews, and scanner recommendations.

Most expensive mistake: approving a catalytic-converter replacement before cheaper causes are ruled out. Some P0420 repairs really are £500+, but many are not.

Typical P0420 Repair Cost in the UK

Likely causeTypical UK costComment
Exhaust leak or gasket issue£60–£180Often the cheapest repair path
Upstream oxygen sensor£90–£220Needs diagnosis, not blind replacement
Downstream oxygen sensor£90–£220Possible, but less often the real cause
Catalytic converter replacement£300–£900+Higher for OEM or awkward-fit vehicles

Why One Garage Quotes £120 and Another Quotes £700

Because the first quote might cover diagnosis and a sensor or leak repair, while the second jumps straight to the catalytic converter. On older UK cars, especially those with rusted exhaust fixings or expensive OEM catalysts, labour and parts can escalate fast. But that still does not mean the cat should be the first assumption.

What to Ask Before Approving Repairs

  1. Was the exhaust checked for leaks first?
  2. Were oxygen sensors tested or just blamed?
  3. Are there related misfire or fueling codes that might have caused the catalyst fault?
  4. Is the quote for OEM or aftermarket parts?
Good repair logic: cheap checks first, then sensor plausibility, then catalyst replacement only when the rest of the system looks healthy and the code still returns.

Will P0420 Fail MOT?

Very often, yes. If the engine management light is on, there is already MOT risk. If the catalytic converter is genuinely underperforming, emissions can fail too. That is why many drivers search this code only after the MOT becomes an urgent problem.

Related guides

→ P0420 full diagnosis guide→ P0420 symptoms UK→ How to fix P0420 yourself→ Catalytic converter replacement cost UK→ Will an engine management light fail MOT?

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Disclaimer: Informational guidance only. Cost ranges vary by vehicle, region, and parts choice.