Ford P0420

Catalyst efficiency below threshold on Ford petrols — on EcoBoost engines this is often a cracked exhaust manifold rather than a worn cat.

Ford P0420 — Catalyst efficiency below threshold on Ford petrols — on EcoBoost engines this is often a cracked exhaust manifold rather than a worn cat. Typical UK independent garage cost: £180–£950.

What actually causes P0420 on a Ford

Ford 1.0 EcoBoost engines have a known issue with the integrated exhaust manifold cracking around the rear cylinder, which causes a small exhaust leak before the cat. The cat then can't reach light-off temperature, post-cat O2 readings drift, and P0420 stores. Drivers also report a faint ticking on cold start. The 1.6 Ti-VCT typically has a genuine cat efficiency drop after 90,000 miles, but always exclude exhaust leaks first.

Ford engines that log P0420

• 1.0 EcoBoost (Fiesta, Focus, B-Max)
• 1.6 Ti-VCT / Duratec
• 1.6 EcoBoost (Focus ST-Line)
• 2.0 EcoBoost (Mondeo, Kuga)

How we diagnose P0420 on a Ford

1. Cold-start the engine and listen for ticking at the manifold — common 1.0 EcoBoost manifold crack tell
2. Smoke-test the exhaust from the turbo downpipe to the rear of the cat
3. Compare pre- and post-cat O2 sensor live data on a scan tool — true cat failure shows post-cat mirroring pre-cat
4. Check for any pending misfire codes (P0301–P0304) that would damage the cat
5. Only replace the cat if manifold, leaks and lambdas all check out clean

The common misdiagnosis to avoid

Many UK Ford specialists immediately quote for a new cat (£500–£700) on these cars without smoke-testing the manifold or testing both lambdas independently. A new cat fitted to a cracked manifold will fail again within months.

Ford P0420 repair cost in the UK

Expect £180–£950 including parts and labour at an independent garage. Main-dealer quotes on Ford are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.

Service campaign / TSB note

Ford issued multiple TSBs covering 1.0 EcoBoost cooling and manifold concerns 2012–2019.

Bob's workshop tip

Ford did extend warranties on some 1.0 EcoBoost coolant and manifold issues — always check if the car has any unactioned Ford service actions on the VIN before you spend a penny.

Codes usually seen alongside P0420 on Ford

• P0300
• P0171
• P0430

Symptoms drivers report with P0420

• Engine warning light on steady, no obvious drivability change day-to-day
• 3–7 mpg drop over the last 2,000 miles with no other explanation
• Faint smell of sulphur / rotten eggs from the exhaust under load
• Muffled rattle inside the cat when you tap it with a rubber mallet (loose substrate)
• MOT emissions test fails on Lambda, CO or HC — the P0420 was already MIL-on at the pre-test
• Sudden loud exhaust roar from cold start = catalytic converter has been stolen, not worn
• Paired codes: P0430 (bank 2), P0171/P0174 (lean), P0300-series misfires, P0136/P0156 (lambda circuit)

Repairs that clear P0420

• Smoke-test exhaust for leaks
• Graph both lambda outputs at steady state
• Replace rear lambda as cheaper trial
• Replace cat with type-approved unit if confirmed

For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0420 guide.