Land Rover P0420

Catalyst efficiency on Land Rover — TDV6 3.0 diesel cat/DPF system and L322 petrol V8 cat failures.

Land Rover P0420 — Catalyst efficiency on Land Rover — TDV6 3.0 diesel cat/DPF system and L322 petrol V8 cat failures. Typical UK independent garage cost: £700–£3500.

What actually causes P0420 on a Land Rover

Land Rover P0420 on petrol V8 is usually genuine cat substrate failure or a melted cat from misfire history — common on high-mileage RR Sport supercharged. On TDV6 diesel, P0420 is rare (DPF differential pressure codes are more typical) but when it appears, the cat-DPF combined assembly is involved and the bill is significant. Always check for crank vibration damper failures on TDV6 which can damage the whole exhaust.

Land Rover engines that log P0420

• TDV6 3.0 diesel (Discovery 4/Range Rover Sport)
• SDV6 3.0 diesel
• 5.0 V8 supercharged petrol

How we diagnose P0420 on a Land Rover

1. Pathfinder / SDD: log front/rear O2 switching and any misfire counters
2. On V8, inspect coil packs and plugs — any misfire history damages cats
3. On TDV6, inspect crank pulley damper and check DPF differential pressure
4. Bore-scope cat substrate through O2 port where possible
5. Always quote OE or Eberspächer aftermarket — bargain cats fail emissions

The common misdiagnosis to avoid

On 5.0 V8 supercharged, never fit a cat for P0420 without first ruling out misfire history — a worn coil pack damages a new cat in hours.

Land Rover P0420 repair cost in the UK

Expect £700–£3500 including parts and labour at an independent garage. Main-dealer quotes on Land Rover 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.

Bob's workshop tip

RR Sport V8 P0420 with melted cat substrate is misfire-driven 90% of the time — fix the misfire root cause first, then cat, or it happens again.

Codes usually seen alongside P0420 on Land Rover

• P0421
• P0300
• P2459

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.