The Mercedes-Benz CLS250 CDI is a four-door fastback executive car that occupied a unique space in the Australian luxury segment from 2011 through to 2018. Built on the C218 platform, it was powered by the 2.1-litre OM651 twin-turbo four-cylinder diesel paired with Mercedes' 7G-TRONIC seven-speed automatic gearbox. The CLS250 CDI offered genuine diesel economy alongside striking styling, but it came with the running costs and repair bills that go hand-in-hand with an ageing European prestige car. Reliability is broadly acceptable when the car has been serviced on time, but examples stretched between services or past 150,000 kilometres can expose some well-documented and expensive weak points.
OM651 engine: timing chain and tensioner
The most serious fault on the OM651 is the timing chain system on earlier builds. Unlike many modern diesels that use a duplex (double-row) chain, the OM651 uses a simplex (single-row) chain. On higher-mileage or poorly serviced examples — particularly pre-2013 production — the chain stretches, guides wear, and the tensioner loses the ability to maintain correct tension. Symptoms include a rattling or clattering noise on cold start that fades once oil pressure builds, rough running, and in severe cases misfires or hard starting. Ignored, a chain failure causes catastrophic engine damage.
The timing chain assembly sits at the rear of the engine, making this a labour-intensive repair. At an independent specialist, budget $2,500–$4,500 for a complete chain, tensioner, guides and sprocket replacement. Address this proactively around 100,000–150,000 km on older builds rather than waiting for a failure.
Fuel injectors: wear and "black death"
Early OM651 injectors — roughly pre-2012 production — had a higher-than-average failure rate and were subject to a Mercedes redesign. Injector wear causes rough running, smoke, and poor fuel economy. A related failure mode occurs when the injector copper seating washer fails: combustion gases leak around the base of the injector, baking hard carbon onto the injector body and cylinder head. Caught early, replacement costs around $600–$1,200 per injector. Left too long, the injectors seize in the head and removal becomes extremely difficult and far more expensive.
When inspecting a used CLS250 CDI, look for black sooty deposits around any injector on the top of the engine — a telltale sign of a weeping seal.
Intake manifold swirl flaps
The OM651 uses swirl flaps in the intake manifold to optimise low-rpm combustion. The actuator motor that drives these flaps is a known failure point, triggering fault codes (commonly P200A) and reducing power. Carbon and soot from the EGR system also coats the intake manifold interior over time, restricting airflow and causing rough idling. A walnut blast or chemical clean is a sensible maintenance item around 100,000–120,000 km. Intake manifold replacement (when the flap mechanism fails mechanically) runs $1,000–$2,000 at a workshop.
DPF and EGR system
The CLS250 CDI's diesel particulate filter (DPF) and EGR valve can both cause issues, particularly on cars used heavily in urban stop-start driving — the usage pattern that prevents DPF regeneration from completing.
- DPF blockage: Triggers a warning light and limp mode. A forced regeneration at a workshop typically costs $200–$400, but a repeatedly blocked DPF may require replacement at considerably greater expense.
- EGR valve fouling: Soot accumulation causes sticking and rough idle. Cleaning or replacement typically falls in the $400–$900 range.
A car with regular highway use and a documented service history carries less risk here than one used predominantly in the city.
7G-TRONIC transmission
The seven-speed 7G-TRONIC automatic is generally durable but not maintenance-free. Pre-2014 examples can develop delayed engagement, hesitation when pulling away, or harsh downshifts — often from degraded fluid or a worn valve body solenoid. Mercedes specifies the fluid as a "lifetime fill," but many specialists recommend a drain-and-fill every 60,000–80,000 km. Valve body repairs typically run $1,500–$3,000; a full rebuild is significantly more. Verify transmission service history on any example above 120,000 km.
AIRMATIC air suspension (if fitted)
Some CLS250 CDI examples were optioned with AIRMATIC adaptive air suspension. Air spring bladders crack and perish over time, and the air compressor can fail — especially if working hard to compensate for slow leaks. Symptoms include uneven ride height, a suspension warning on the instrument cluster, or a ride that feels firmer than expected. Rear air strut replacement runs roughly $1,000–$2,000 per corner at an independent specialist. Before buying, confirm ride height is even at all four corners and the system responds normally.
COMAND infotainment and electrical faults
The COMAND infotainment system is a common source of frustration as these cars age. Screen delamination — where the display develops a bubbling or cloudy appearance — affects older units, and the system can freeze on boot or lose Bluetooth pairing. Replacement of the COMAND head unit can cost $800–$2,500 depending on whether a used or remanufactured unit is sourced.
A broader electrical quirk worth knowing: a weak or ageing 12V battery can trigger clusters of unrelated warning lights (ABS, ESP, SRS). Replacing and correctly registering the battery with the car's electronics resolves this, but the misfire of warnings can mislead owners into unnecessary diagnostic spending.
Recalls and safety notices
The CLS250 CDI was included in Australia's compulsory Takata airbag recall. The ACCC took Mercedes-Benz Australia to the Federal Court over its handling of the recall and communications with owners, resulting in a $12.5 million penalty — so this is a recall to take seriously. If you are considering a 2018 example, check whether the airbag recall has been completed via the 2018 CLS250 recall page on Carify. For a broader overview of recall activity, visit the Carify recalls page.
Buying a used Mercedes-Benz CLS250 CDI? What to check
- Cold-start chain rattle: Start from cold and listen for rattling or tapping in the first 10–20 seconds. A noise that fades as oil pressure builds points to timing chain wear.
- Injector seals: Look for black carbon deposits around the base of any injector — a weeping seal means injector work is imminent.
- Oil change frequency: The OM651 rewards short service intervals. Look for changes no more than 15,000 km apart; 10,000 km is better on older examples.
- DPF and EGR history: Ask about any DPF warning lights or forced regens. A scan tool check for stored fault codes will reveal the picture quickly.
- AIRMATIC (if fitted): Check ride height at all four corners and listen for the compressor running at start-up.
- Transmission: Drive from cold — hesitation or harsh shifts may indicate valve body wear or degraded fluid.
- Recall status: Run a Carify VIN check to confirm the Takata recall has been completed and review the vehicle's full history.
A pre-purchase inspection by an independent Mercedes-Benz specialist is money well spent. Factor in ongoing annual maintenance of $800–$1,800 at an independent specialist for routine servicing, with diesel-specific repairs adding considerably more when needed. Before committing, also run a PPSR check to confirm no finance or write-off history.
The verdict
The Mercedes-Benz CLS250 CDI is a genuinely appealing used buy — a distinctive executive fastback with real diesel economy and a refined driving character. But it is not a low-cost ownership proposition. The OM651 engine has several well-documented failure points that become progressively more likely as kilometres accumulate, and repair bills from a Mercedes specialist carry real weight. The best examples have documented service histories with regular oil changes and verifiable recall completion. Browse the Carify car problems and recalls hub to compare this model against others in the segment before you decide.