What Mercedes-Benz codes (P0299, P2002, P0420, P0401, P0203, P0106) actually mean on Mercedes-Benz engines — the real cause, the common misdiagnosis, and the honest UK repair cost.
These are the OBD fault codes we see most often on Mercedes-Benz vehicles in the workshop, with the real cause on Mercedes-Benz engines rather than the generic textbook definition, plus the honest independent-garage cost.
Mercedes-Benz fault codes explained
- P0299 — Turbo underboost on Mercedes — on 2.1 OM651 diesel it's the boost pressure sensor or charge air pipework; on M271 petrol it's the supercharger or chain. Typical UK cost £180–£2200.
- P2002 — DPF efficiency on Mercedes diesels — OM651 and OM642 engines suffer carbon-blocked EGR cooler that fouls the DPF prematurely. Typical UK cost £280–£2400.
- P0420 — Catalyst efficiency on Mercedes petrol — on M271 1.8 Kompressor, the supercharger oil seal failures fouls the cat. On M276 V6, lambda sensor drift is more common. Typical UK cost £180–£1400.
- P0401 — EGR flow insufficient on Mercedes OM651/OM642/OM646 diesels — typically a stuck EGR valve or cooler clogging on city-driven C/E/Sprinter. Typical UK cost £260–£1100.
- P0203 — Injector circuit Cylinder 3 on Mercedes — OM651/OM642 piezo injectors and OM611/OM646 Delphi units both fail with mileage; cyl 3 often singled out by harness routing. Typical UK cost £380–£1300.
- P0106 — MAP sensor performance on Mercedes — OM651 diesel boost sensor and M271/M274 petrol intake sensors both fail with mileage. Typical UK cost £140–£480.
- P0562 — System voltage low on Mercedes — alternator failure and dual-battery system issues on S/E/GL with second battery. Typical UK cost £180–£720.
- P0100 — MAF circuit malfunction on Mercedes — OM651 diesel MAF and M271/M272/M276 petrol MAF all fail with mileage and EGR contamination. Typical UK cost £160–£520.
- P0304 — Cylinder 4 misfire on Mercedes — M271 Kompressor and M274 turbo petrol coil failure, OM651 diesel injector or glow plug. Typical UK cost £160–£720.
- P0171 — System too lean on Mercedes — M271 1.8 Kompressor and M274 2.0 turbo both have a signature intake manifold runner / PCV issue. Typical UK cost £160–£620.
- P0301 — Cylinder 1 misfire on Mercedes — M271/M272/M274 coil packs, injectors and timing chain wear are the headline causes. Typical UK cost £100–£1800.
- P0410 — Secondary air injection malfunction on Mercedes — M271 Kompressor SAI pump is the textbook UK failure. Typical UK cost £250–£1400.
- P0193 — Fuel rail pressure sensor high on Mercedes — OM651/OM642 CDI rail sensor failures, sometimes CP4 related. Typical UK cost £180–£1200.
- P0430 — Bank 2 cat efficiency on Mercedes V6/V8 — M272/M273 balance-shaft gear wear or M276 timing chain causing upstream raw fuel. Typical UK cost £300–£3200.
- P0087 — Fuel rail pressure too low on Mercedes OM651/OM642 diesel — HP pump shaft wear (Bosch CP4) and fuel contamination. Typical UK cost £600–£6000.
- P0011 — VVT 'A' over-advanced Bank 1 on Mercedes — M271 magnet adjuster failure and M276 chain stretch. Typical UK cost £140–£2400.
- P0016 — Crank/cam correlation Bank 1 on Mercedes — M271 chain stretch (single-cam) and M276 stretched chain. Typical UK cost £600–£2600.
- P0089 — Fuel pressure regulator on Mercedes — OM651/OM642 DRV and CP4 pump regulator failure. Typical UK cost £250–£6000.
- P0234 — Turbo overboost on Mercedes — OM651 BiTurbo low-pressure actuator and M274 wastegate stick. Typical UK cost £240–£1600.
- P0507 — Idle speed higher than expected on Mercedes — M271/M274/M276 PCV diaphragm and vacuum hose. Typical UK cost £180–£580.
- P2463 — DPF soot accumulation on Mercedes — OM651 / OM642 DPF over-soot from failed regens. Typical UK cost £180–£2400.
- P0341 — Camshaft position sensor performance on Mercedes — M271/M272/M274 sensor failure with intermittent stall. Typical UK cost £140–£420.
Codes are generic across manufacturers, but the fix is not. The same code on a Mercedes-Benz usually points at a different component than it would on another make, which is why a make-specific diagnosis saves money before any parts are bought.
Reading codes properly on a Mercedes-Benz
A £20 reader pulls generic engine codes and nothing else. On a Mercedes-Benz the useful information often sits in the gearbox, ABS, body or comfort modules, and those need a tool that talks the manufacturer protocols. Just as important is the freeze-frame data stored with the code: engine speed, load, coolant temperature and fuel trims at the exact moment the fault was logged. That snapshot is usually what separates a failing sensor from a real mechanical problem, and it is the first thing we look at before anything is unbolted.
Why the same code costs different money on a Mercedes-Benz
Each code on this page has a cost range rather than a price because the cheap cause and the expensive cause log identically. A split hose and a failed turbo can both set a boost code. Access matters too — some Mercedes-Benz engines put the awkward part behind half the front end, and the labour, not the part, is what moves the bill. An honest quote names which cause has been confirmed and what the repair for that specific cause costs.
Getting a Mercedes-Benz fault diagnosed
Bob's Mechanical Repairs is an independent, family-run garage in Birnam, Dunkeld, Perthshire, a couple of minutes off the A9. Bob has 32 years on the tools, and diagnostics are charged as diagnostic time with the findings explained in plain English and the repair priced before any work is agreed. An MOT is £54.85 if the car is due one at the same time.
Common questions
How do I read fault codes on a Mercedes-Benz? Any generic OBD-II reader will pull engine codes from a Mercedes-Benz built after 2001, but manufacturer-level codes from the gearbox, ABS and body modules need a tool that speaks the Mercedes-Benz protocols. That is why a garage scan often finds codes a cheap reader misses.
Will clearing a Mercedes-Benz fault code fix it? No. Clearing the code only erases the record. If the fault is still present it returns on the next drive cycle, and in the meantime the readiness monitors are reset, which can stop the car passing an emissions check.
How much is a Mercedes-Benz diagnostic at an independent garage? An hour of diagnostic time is typically £50–£90 in the UK, against £110–£180 at a main dealer. It almost always costs less than the part it stops you buying by mistake.