Smart ForTwo
The Mitsubishi three-cylinder in the ForTwo 451 is fundamentally a solid engine. In naturally aspirated form it is very frugal and dependable; the turbo version offers noticeably more driving fun at a slightly higher maintenance requirement.
Everyday open Smart
71 hp MHD in the cabrio — enough for the city, open-top motoring puts a smile on. Many owners would choose the turbo in hindsight.
Engine Weaknesses 9
The timing chain tensioner guide consists of two parts — the yellow sliding coating dissolves, causing a loose timing chain and in the worst case valve damage. Mainly affects engines with neglected oil changes.
Symptoms: Rattling on cold start (disappears after about 1 second), engine warning light, sudden power loss
The shaft of the electric motor in the clutch actuator breaks from around 100,000 km. The Softtouch gearbox shifts with jerks or not at all. Display shows only 3 bars, no gear can be selected.
Symptoms: Jerky gear changes, transmission limp mode, '3 bars' shown on display, no gear selectable, fault codes P0120/P0222/P0123
Faulty ignition coils cause misfires on individual cylinders. With a 3-cylinder engine even one failure is very noticeable. Cylinders 2 and 3 are most frequently affected.
Symptoms: Engine warning light, pronounced jerking, power loss, rough idle, misfires on individual cylinders
On MHD models the starter-generator bracket bends, causing the pulley and V-belt to run out of alignment. The belt snaps, often preceded by a chirping noise that varies with engine speed as a warning.
Symptoms: RPM-dependent chirping/squealing, broken V-belt, battery warning light, alternator non-functional
The wiring harness at the starter corrodes within the first few centimetres. Oxidation on connectors and earth connections causes starting failures. Up to 2009 the starters themselves were also failure-prone.
Symptoms: Engine won't start despite a full battery, weak clicking noise when attempting to start, intermittent starting problems especially in damp conditions
The Smart has no self-bleeding cooling system because the coolant lines run through the underfloor lower than the engine. After any time the circuit is opened, air pockets risk overheating.
Symptoms: Temperature gauge rises quickly, coolant warning light, engine overheats after a short drive, heater blows cold despite warm engine
Oil loss between the valve cover and cylinder head is common on the 3B21, especially with short-trip use. The vulcanised-on gasket hardens over time.
Symptoms: Oil stains on the engine casing, oil smell at the rear, visible oil film between valve cover and cylinder head, may flag at MOT
The lambda sensor heater element can fail in under 20,000 km. Only 10.5 V instead of the required 12 V reaches the sensor, causing faulty mixture control and the engine warning light.
Symptoms: Engine warning light, fault code 'Bank 1 heater fault', increased fuel consumption, fault may disappear temporarily after clearing
The torque strut on the front engine mount goes soft or fractures. Responsible for roughly 90% of all vibration complaints on the 451. Costs only about €30 and swapped in 10 minutes.
Symptoms: Heavy vibrations at idle, jerking under load changes, noticeable shaking inside the cabin, clunking over bumps
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.