Smart fortwo 1.0 W451

The five-speed Softtouch automated manual gearbox uses a clutch actuator that transfers rotational movement to linear push via a spiral gear. This gear wears prematurely, particularly when the factory grease dries out and crystallises. Symptoms include increasingly jerky gear changes, inability to select certain gears, sudden loss of drive, and dashboard gear warnings. The actuator is coded to the vehicle, meaning a new unit must be electronically paired. At a Smart/Mercedes dealer, replacement including coding typically runs €800-1,500. Independent Smart specialists and remanufacturers offer repaired units from €300-600 fitted, sometimes with lifetime warranties and no re-coding required. This is the single most common expensive repair on the W451. It can happen from as early as 40,000 km, though 60,000-100,000 km is more typical.

On MHD (Micro Hybrid Drive) models, a single auxiliary belt drives the water pump, alternator, and starter-generator. If this belt snaps while driving, the water pump stops immediately and the engine can overheat catastrophically within two minutes. Evilution, the primary Smart technical resource, documented the first 451 belt failure in 2010, and it has become a recognised common problem since. Cases have been reported as early as 10,000 km. If you stop immediately when the warning lights illuminate, the repair is just the cost of a new belt (€30-50 plus labor). If you continue driving and overheat the engine, head gasket failure or warped cylinder head can push costs to €2,500-3,500 for engine replacement. Preventive replacement every 40,000-50,000 km or every 4 years is strongly recommended. Non-MHD models have a conventional alternator belt that is less critical but should still be inspected regularly.

The front suspension anti-roll bar bushes are highly susceptible to water and salt ingress. According to Evilution, 9 out of 10 suspension noise complaints on the 451 are caused by this issue. The rubber bushes absorb moisture and then crack, producing creaking, cracking, and banging noises over bumps. This typically starts from 40,000-60,000 km, or sooner in regions with salted winter roads. OEM rubber bush replacement costs €100-200 including labor. Polyurethane replacements from brands like Powerflex offer better durability at a similar price. Full control arm replacement, if the bushes are integrated, runs €200-400 per side.

The rear-mounted engine uses rubber mounts that are known to split and deteriorate. When the mount fails, the engine moves excessively under acceleration and braking, causing noticeable vibration and clunking. The design is considered poor, with the mount simply not robust enough for the engine's torque pulses. A new OEM mount costs €40-60, with aftermarket and performance options ranging from €90-180. Labor is relatively straightforward at around one hour. Most owners encounter this between 60,000-120,000 km. Some owners choose to repair the original mount rather than replace it, as upgraded aftermarket mounts can transmit more noise into the cabin.

The Signal Acquisition Module (SAM unit), essentially the main fuse box and relay board, can develop cracked or poor solder joints from vibration over time. This causes a wide range of intermittent electrical symptoms: no-start conditions, dashboard warning lights coming on randomly, immobiliser faults, clicking noises near the fuse box with no dash lights, and various CAN communication errors. Many owners have successfully repaired this by re-soldering the affected joints, which can be done by an electronics specialist for €100-200. A replacement SAM unit from Smart costs €300-500 plus coding. The issue is particularly common in cars over 8-10 years old.

City cars driven predominantly on short trips never fully heat the exhaust system, allowing moisture and acid condensation to accelerate internal corrosion. The Smart's low ground clearance also exposes the exhaust to road salt and debris. In northern European climates, exhausts can rust through from as early as 6-7 years. A replacement aftermarket exhaust section typically costs €150-350, with fitting adding €100-250 depending on how corroded the mounting hardware has become. Regular longer drives to fully heat the exhaust system help extend its life.

The visible tridion safety cell has good powder-coat protection and usually resists corrosion well. However, the engine subframe, front steering knuckles, rear wishbones, and steel bumper reinforcements have far less protection and can corrode significantly, especially on cars that have spent their lives in salt-heavy northern European climates. Rear wheel arches and the area around wheel arch liner fixing screws are particularly vulnerable. Surface rust is often cosmetic, but structural corrosion of the subframe can require expensive welding repairs (€500-1,500) or make the car uneconomical to repair. Underbody inspection before purchase is essential.