Diesel · 2011–2020
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All about the Mercedes-Benz SLK / SLC (R172)All about the 1.8 / 2.1 CDI (Mercedes OM651) engine
The 2.1 CDI (Mercedes OM651) engine was fitted to the Mercedes-Benz SLK / SLC (R172) (2011–2020). In the EU that is 32.3 % of all new cars of this generation.
Versions: 2.1, 2.1 204 hp.
The most expensive known risk is timing chain stretch or tensioner wear (single-row chain at the rear): it usually appears at 120,000–250,000 km and costs €300–2,500 to fix.
✓ The faults of this engine were checked against sources (10/2026). Each item has at least two independent sources or an official recall or parts-maker technical note.
Signs: Engine light, limp mode, rough running and jerking, hard starting, smoke
Fix: Have a dealer check by VIN whether the conversion to solenoid injectors (new ECU software, return line) was done; otherwise test and replace the faulty piezo injectors
Signs: Harder starting, rough idle, knocking, smoke, diesel smell or soot around an injector
Fix: Return-flow test and check of the quantity correction values to find the weak injector; replace that injector or its seal rather than the whole set
Signs: Rattle for a few seconds after a cold start, later constant; P0016 / P0017, power loss
Fix: Read the cam-crank phase offset and measure chain stretch; fit the revised tensioner if the chain is in tolerance, otherwise replace chain, sprocket, tensioner and guides
Signs: Usually none before failure; oil seepage at the tensioner, in extreme cases the tensioner loosens and the chain can jump
Fix: Have a dealer check by VIN that the free seal ring replacement was done (about 1.5-2.5 h); also applies to older cars that got a tensioner from that period
Signs: Falling coolant level, traces near the intake manifold or fuel filter, temperature warning
Fix: Pressure-test the system; replace the coolant flange with the reinforced version (EGR cooler must come off) or the water pump; ask the dealer about service action 1204P59A93
Signs: Oil on the front of the engine and under the car, oil smell; rarely oil and coolant mixing
Fix: Find the exact leak (oil cooler check valve O-ring, transfer pipe seals or the cooler itself) and replace the seals or the oil cooler
Signs: Slight to clear power loss, P0401 (low EGR flow), rough idle, black smoke; a cracked cooler loses coolant into the intake
Fix: Compare EGR target and actual position in live data; clean the valve thoroughly or replace it, and the cooler if it is blocked or leaks
Signs: DPF warning, frequent or interrupted regenerations, limp mode
Fix: Check the pressure sensor and soot load first and fix any EGR or injector fault; forced regeneration, cleaning or replacement; use MB 229.51 / 229.52 low-ash oil
Signs: Engine light with boost pressure or flap codes (e.g. P200A), power loss, rough idle
Fix: Smoke-test the intake for cracks and check the flap motor; replace the manifold with a new flap motor (coolant must be drained)
Signs: No symptoms; an open recall can lead to registration problems, some owners report slightly higher fuel use after it
Fix: Have a dealer check by VIN that the free software update was done
Signs: Slipping, high bite point
Fix: Replace clutch kit
Signs: Holds a gear too long then shifts hard, sticks in gear or limp mode, converter lock-up stops working; codes 0717 / 0718 / 0722 / 2201
Fix: Confirm with Mercedes-level diagnostics; replace or rebuild the conductor plate (with the valve body on early VGS1 units), change the fluid with the correct MB spec and code / adapt the unit
Signs: Harsh shifts when warm, drops from D to N at stops, starts in 3rd, reverse will not engage, 'F' on the dash; codes P073E00, P07B700
Fix: Read the codes and fluid condition; repair / rebuild the mechatronic (pressure sensors) or replace it, then change the fluid and run the adaptations
Typical routine maintenance cost for 100,000 km: €2,400–5,750. Sum of the services and wear parts in the plan above (about 15,000 km a year, independent workshop), without faults.
Intervals and typical prices for the Mercedes-Benz SLK / SLC (R172) with the 2.1 CDI (Mercedes OM651) engine. The app gives the exact plan for your mileage.
| Item | Interval | Typical price |
|---|---|---|
| DPF health check (soot / ash load)Emissions | every 20,000 km | €0–60 |
| Engine oil + oil filterEngine | every 15,000 km or every 12 months | €70–150 |
| Cabin (pollen) filterClimate | every 15,000 km or every 12 months | €15–40 |
| Brake fluidBrakes | every 24 months | €50–95 |
| A/C service (regas + desiccant)Climate | every 24 months | €70–150 |
| Air filterEngine | every 30,000 km or every 24 months | €15–45 |
| Fuel filterFuel | every 30,000 km or every 24 months | €40–110 |
| Glow plugsEngine | every 100,000 km | €120–300 |
| Auxiliary (serpentine) beltEngine | every 120,000 km | €70–180 |
| Coolant replacementCooling | every 150,000 km or every 96 months | €70–150 |
| Timing chain inspection (listen on cold start)Engine | every 150,000 km | €0–120 |
| Manual gearbox oilTransmission | every 150,000 km | €60–130 |
| Front brake padsBrakes | usually at 30,000–60,000 km | €60–150 |
| Front brake discs + padsBrakes | usually at 60,000–120,000 km | €150–320 |
| Rear brake padsBrakes | usually at 50,000–90,000 km | €50–130 |
| Tyres (set of 4)Wheels | usually at 40,000–60,000 km | €320–700 |
| Wiper bladesBody | checked at service | €20–50 |
| 12V batteryElectrical | checked at service | €100–220 |
| Version | Share | Years | CO₂ | Fuel use |
|---|---|---|---|---|
| 2.1 Diesel2,143 cm³ | 24.7 % | 2012–2018 | 114 g/km | — |
| 2.1 Diesel2,143 cm³ · 150 kW (204 hp) | 7.6 % | 2012–2019 | 114 g/km | — |
Source: European Environment Agency (EEA), monitoring of CO₂ emissions from new passenger cars, CC BY 4.0. CO₂ and fuel use are official type-approval values (WLTP from 2021), not real-world consumption.
The faults with the highest risk: timing chain stretch or tensioner wear (single-row chain at the rear) (120,000–250,000 km, €300–2,500); DPF clogging (mostly short trips) (80,000–200,000 km, €150–1,800); injector wear or leaking injector seals (solenoid injectors) (150,000–300,000 km, €150–1,500); EGR valve and cooler coking (100,000–220,000 km, €300–1,500).
Chain.
5W-30 · MB 229.51 / 229.52 · ~6.5 L · every 15,000 km.
1.6 / 1.8 Kompressor / CGI (Mercedes M271)1.6 / 2.0 petrol (Mercedes M274)3.0 / 3.5 V6 petrol (Mercedes M276)
Mercedes-Benz C-Class (W205) 2014–2021Mercedes-Benz C-Class (W204) 2007–2014Mercedes-Benz E-Class (W212) 2009–2016Mercedes-Benz A-Class (W176) 2012–2018Mercedes-Benz GLC (X253) 2015–2022Mercedes-Benz CLA (C117) 2013–2019Mercedes-Benz GLA (X156) 2013–2020Mercedes-Benz B-Class (W246) 2011–2018Mercedes-Benz Vito (W447) 2014–Mercedes-Benz ML / GLE (W166) 2011–2019Mercedes-Benz GLK (X204) 2008–2015Mercedes-Benz CLS (C218) 2011–2018
Main sources for the faults of this engine:
Faults, intervals and prices are indicative and collected from public sources and owner experience; always rely on the service book and a qualified workshop. Test statistics: DVSA – Anonymised MOT tests and results, Open Government Licence v3.0. Dutch tests: RDW Open Data (CC0). Engine versions: EEA, monitoring of CO₂ emissions from new passenger cars (CC BY 4.0, DG CLIMA). Owner complaints: NHTSA ODI (public domain). Recalls: DVSA Vehicle Safety Branch Recalls Database, OGL v3.0. Brand and model names are trademarks of their owners and are used only to describe the vehicles. How we check the data →
Updated 29/10/2026