Smart ForTwo
The smallest production diesel of its time. Fundamentally unsuited to city driving and short trips — precisely the use case of most Smart owners. DPF problems, turbo wear and high repair costs relative to vehicle value make the CDI an engine to avoid. The petrol engines are the far better choice.
Open-top clatter
45 hp CDI as a cabrio — the engine is too weak and too noisy to benefit from the open roof. A pure rational purchase.
Engine Weaknesses 4
On Euro 5 models (54 hp) a sensor was incorrectly positioned. In winter, ice can form in the intake tract and damage the turbocharger. Limp mode activates.
Symptoms: Engine limp mode in winter, power loss, whistling noises from the turbocharger, engine warning light
The injectors of the OM660 coke up through frequent short-trip use. At 150,000 km and beyond, cold-start problems and rough running appear.
Symptoms: Poor cold start, rough running, power loss, increased consumption, heavy smoke
The DPF on the Smart CDI clogs frequently because the vehicle is typically used for short trips. Regeneration requires 20–30 minutes of sustained driving at around 120 km/h.
Symptoms: Power loss, DPF warning light, increased consumption, engine enters limp mode, regeneration does not complete
The EGR cooler on the OM660 develops leaks and loses coolant. The coolant enters the intake tract and can damage the engine.
Symptoms: Dropping coolant level, white smoke from exhaust, coolant smell, possible engine overheating
Vehicle Weaknesses 13
The shift actuator (part no. A4518290201) controls the electronic gear selection. When it fails, individual gears are lost or the gearbox no longer shifts at all. Typical problem on older examples from 100,000 km.
The SAM control unit in the driver's footwell is susceptible to moisture ingress via leaking windows or door seals. Corroded solder joints (lead-free solder in early production years) cause failures. Vehicle does not start, warning lights flicker.
The 451 electric power steering is prone to failure due to worn carbon brushes or a stuck relay contact. In the worst case, the complete steering rack must be replaced. Sudden failure while driving is possible.
Recall of 90,889 vehicles (model years 2010–2014) due to possible short circuit in the heater shut-off valve. Vehicles where the recall was not carried out may pose a fire risk. Recall should have been performed free of charge at a workshop.
On the early 451 (before the 2010 facelift), the catalytic converter and rear silencer are a single unit. Typical short-trip use means condensation water never dries out, and the steel rusts from the inside. Replacement including fitting costs €600–1,200.
The 451 instrument cluster can show pixel failures or fail completely. Specialist repair services offer solder joint repairs. The instrument cluster is not just a display but also a control unit.
On early production years (up to 2009), faulty rain sensors caused parasitic drain that discharged the battery overnight. Mercedes resolved the problem by switching suppliers. Older examples may still be affected.
As a typical city car, the 451 is rarely driven far enough to warm through. Moisture remains under the plastic Tridion frame and promotes corrosion on suspension components, brake lines and the exhaust system. MOT inspectors flag this frequently.
On the 451 cabriolet, the rubber seals between the soft top and bodywork wear prematurely. Particularly at the A-pillar at the front, gaps of up to 5 mm develop through which water enters the footwell in rain.
The front drop links (part no. A4513200189) wear out and rattle over bumps. A frequent MOT advisory. Cheap to repair but often overlooked.
The 451 air conditioning is prone to slow leaks. Many owners report having to top up the refrigerant annually. Older vehicles with a defective compressor or condenser can incur repair costs of €300–1,000.
The tailgate lock can jam and refuse to open. A typical comfort issue at higher mileages. Affects both coupé and cabriolet. Cheap to fix, but annoying.
The 451 cabriolet suffers from strong wind noise above approx. 80 km/h with the top closed. Causes include worn slide rails, incorrectly adjusted hood height, or shrinkage of the soft-top fabric.