Hyundai P0420

Catalyst efficiency on Hyundai — on 1.6 GDI petrol (Theta II family) the famous engine bearing failures damage the cat with debris.

Hyundai P0420 — Catalyst efficiency on Hyundai — on 1.6 GDI petrol (Theta II family) the famous engine bearing failures damage the cat with debris. Typical UK independent garage cost: £180–£1300.

What actually causes P0420 on a Hyundai

Theta II GDI engines (1.6 / 2.0 / 2.4) have a known crank bearing failure issue — Hyundai issued a service campaign in many markets. Bearing wear sends metal particles through the engine and they end up in the cat. Always check for any engine noise (rod knock) before authorising any cat work on these.

Hyundai engines that log P0420

• 1.6 GDI (i30, i40, Tucson)
• 2.0 GDI (Tucson, Santa Fe)
• 1.6 CRDi (i30, i40, ix35)

How we diagnose P0420 on a Hyundai

1. Listen for cold-start rod knock — diagnostic of bearing failure
2. Check for any open Hyundai service campaigns on the VIN
3. Test pre/post cat lambdas
4. Compression and leakdown test if engine noise is present

The common misdiagnosis to avoid

Don't replace the cat on a Theta II GDI without checking for engine bearing knock — if the engine is failing, the new cat will be destroyed by the next round of debris.

Hyundai P0420 repair cost in the UK

Expect £180–£1300 including parts and labour at an independent garage. Main-dealer quotes on Hyundai 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

Hyundai-Kia Theta II engine service campaign — check VIN.

Bob's workshop tip

If you're out of warranty with a knocking Theta II engine, an engine swap is often more economical than a rebuild plus new cat. Used Theta II engines are widely available.

Codes usually seen alongside P0420 on Hyundai

• P0430
• P0300
• P0301

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.