IsItAGoodCar
Reliability Guide

Common Problems with the
2008 Smart Fortwo (451)

The 451 Fortwo arrived in 2007 with a 1.0-litre Mitsubishi-sourced engine, a significant improvement over the fragile 450. The 2008 MHD (Mild Hybrid Drive) models introduced a belt-driven starter-generator, but this belt can snap and cause instant overheating. Ensuring the updated belt kit has been fitted is essential.

What changed for 2008

  • •New 1.0-litre Mitsubishi engine — more robust than 450 units
  • •MHD (Mild Hybrid Drive) introduced
  • •Automated manual gearbox carried over
  • •Improved safety and refinement over 450

Common MOT watch-outs

  • !MHD drive belt failure — engine overheats almost instantly
  • !Clutch actuator failure on automated manual
  • !SAM unit corrosion from water ingress
  • !Air conditioning pipe chafing and leaking
3
Known Issues
2
Serious
1
Moderate
0
Minor

All Known Issues

MHD Drive Belt Failure

SeriousEngine

The Mild Hybrid Drive system uses a belt to start the engine and drive the water pump. If the belt snaps, the engine will overheat almost instantly, often warping the head.

Est. Repair Cost: £200 (belt) to £2,500 (engine rebuild)
Warning Signs:

Battery light on dash, Engine temperature warning, Sudden loss of power

What to Inspect:
  • Ensure the "Modified" belt kit and tensioner have been installed (Service Campaign work)
Verified by IsItAGoodCar Research Team

SAM Unit Corrosion (Water Ingress)

SeriousElectrics & Electronics

In the UK, water can enter the cabin via the door seals or heater intake and track down to the SAM unit (located in the passenger footwell).

Est. Repair Cost: £500–£1,000
Warning Signs:

Lights coming on by themselves, Dead battery, Non-start issues

What to Inspect:
  • Check for moisture under the passenger side carpet
Verified by IsItAGoodCar Research Team

Clutch Actuator Failure

ModerateGearbox & Transmission

The actuator that operates the clutch can seize or wear out, leading to jerky gear changes or failure to select drive.

Est. Repair Cost: £300–£600
Warning Signs:

Jerky pull-away, Slow gear changes, Car stuck in neutral with "3 bars"

What to Inspect:
  • Listen for a "creaking" noise from under the car when the ignition is turned on
Verified by IsItAGoodCar Research Team

Buying Tips for 2008 Smart Fortwo (451)

  • ✓On MHD models, verify the modified belt kit and tensioner have been installed (Service Campaign work)
  • ✓Test the air conditioning thoroughly — pipes are prone to chafing
  • ✓Look for evidence of "clutch teaching" or adaptation in service history
20072009–2014
Check a 2008 Smart Fortwo (451) by Registration