Smart fortwo 0.8 CDI W450

The automated manual (softip/softouch) gearbox relies on a clutch actuator with internal plastic gears that wear over time. Symptoms include progressively jerkier gear changes, inability to select reverse, delayed engagement, or the car refusing to move entirely. Wiring between the actuator and the intercooler cradle is also prone to chafing, creating intermittent faults. Failure typically occurs between 60,000 and 120,000 km. Specialist repair of the internal gears is possible for around €250-400, while a replacement actuator costs €350-600 for the part. A full gearbox rebuild including clutch disc and actuator can reach €1,200. Keeping the gearbox software updated and having the clutch relearned every 30,000 km can delay failure.

The KP31 turbocharger uses a vacuum-operated wastegate. The 3 mm vacuum hoses feeding the wastegate actuator become brittle and crack over time, preventing proper boost control. The wastegate actuator itself can seize or its internal gasket can leak, causing the wastegate to stick partially open. Symptoms include sudden power loss, increased fuel consumption, and sometimes limp mode without a check engine light. Replacing the vacuum hoses and solenoid costs around €200-300. A full wastegate actuator replacement and recalibration runs €700-950. The turbo itself is generally durable, with many examples running past 150,000 km on the original unit.

Despite the non-rusting plastic body panels, the underlying steel tridion safety cell, subframe, and sills can develop serious corrosion. The W450 left the factory with a very thin wax coating for rust protection. The rear wheel arch area is worst affected: elongated holes for the arch liner fixing studs allow water sprayed by the wheels to penetrate behind the liner, where it sits and rusts through the studs and the metal underneath. Door sills hidden behind plastic mouldings accumulate moisture and rot from the inside. On cars from salt-road regions, structural rust can make the car uneconomical to repair. Professional repair costs €300-500 per area, but extensive rust affecting the tridion cell can exceed the car's value.

The EGR valve accumulates soot from exhaust gases, especially on cars driven mostly in urban traffic. A stuck-open valve causes rough idle and poor performance. A stuck-closed valve can trigger fault codes. The vacuum lines feeding the EGR can also perish and leak, causing erratic EGR behavior. Cleaning the EGR valve is sometimes sufficient at €100-200 including labor. If the valve body is corroded or the solenoid has burned out, full replacement costs €400-500 including the labor needed to access it, which requires bumper removal and some suspension disassembly.

The SAM (Signal Acquisition Module) is the central electronic control unit managing all the car's electrical systems. Water enters through three main routes: a poorly bonded windscreen, a leaking bulkhead seal, or a blocked wiper bucket drain. Once moisture reaches the SAM connectors (particularly N11-8 and N11-9), corrosion causes partial failures: intermittent hazard lights, non-functioning windows, central locking glitches, fuel pump relay faults, or even the immobiliser refusing to deactivate. A new SAM unit from Mercedes with coding costs €600-800. Specialist refurbishment and cloning services are available in Germany for €200-400. Applying dielectric grease to the connectors at first signs of trouble can prevent further damage.

A known design flaw on the W450 CDI: the intercooler is positioned so that the plastic scoop touches the top edge of the intercooler core. Vibration gradually wears through the aluminium fins, creating a boost leak. Symptoms include gradual loss of power and oil dripping from the intercooler area. The fix involves replacing or repairing the intercooler (€100-300 for an aftermarket part) and trimming or modifying the plastic scoop to prevent recurrence. Total repair including labor is typically €150-400.

The OM660 uses a timing chain rather than a belt. The chain itself is hardened and durable, but the sprockets are made from softer iron and show wear over time. The mechanical chain tensioners (not hydraulic) use plastic arms that can snap in cold climates, leaving the chains untensioned. Symptoms start with a ticking noise that progresses to a rattle. If ignored, the chain can wear through the soft aluminium timing cover causing oil leaks, and eventually skip a tooth leading to valve-to-piston contact. This issue typically appears above 150,000 km with regular servicing, or earlier if oil changes have been neglected. Some owners treat replacement as preventive maintenance at high mileage.

A documented but uncommon failure mode on the OM660: exhaust valve guides wear prematurely due to material choice, while inlet valves remain fine. Once worn, valves become sticky, then burn, and in the worst case collide with the piston and destroy the engine. Some owners have reported valve guide wear as early as 80,000 km, though this appears linked to poor maintenance or overheating. The cylinder head must be replaced if valves have burned, as simply replacing the affected valve leads to repeat failure within weeks. A replacement head from Mercedes costs around €800-1,000, plus €200-500 labor. A used engine is sometimes more economical.