P0805 Clutch Position Sensor / Actuator on Smart Fortwo 451
P0805 on a 451 (clutch position sensor / actuator) is usually the actuator failing. Commonly paired with the three-bar warning and a squeaky noise on shifts. Shop fix with MB Star coding.
Typical Symptoms
- Check engine light with code P0805
- Squeaky or scratchy noise when starting in gear
- Slipping clutch on downshifts
- Three-bar transmission warning intermittent or persistent (see related code below)
- Hard or rough gear changes
- Often appears past 100,000 miles
What it means
P0805 is "Clutch Position Sensor Circuit Range/Performance" — the ECU sees the clutch position reading from the actuator and decides the values don’t make sense. On a 451 with the automated manual transmission, the clutch isn’t something you operate — the clutch actuator does it electrically. So P0805 almost always points back at the actuator itself.
Common pattern: owner sees P0805 + a squeaky/scratchy noise when starting + slipping on downshifts, all around 100,000+ miles. Fix runs about $1,000 at an indie shop and they often find the clutch is still fine — just the actuator.
A separate failure mode worth knowing: water ingress. The 451 clutch actuators are more prone to water ingress and failure than the 450 actuators. If you see P0805 after heavy rain or a car wash, that’s why. Check the actuator area for water staining.
Likely causes (cheapest first)
- Battery weak / low voltage during shifts. Worth ruling out first. A marginal battery causes voltage dips that throw the actuator’s position reading off. Test the battery; replace if borderline.
- Actuator adaptation drifted out of range. As the clutch wears, the actuator runs further from its taught position. A clutch / actuator adaptation procedure (MB Star, AutoFlash, Xentry) can buy time.
- Actuator mechanical failure. The dominant cause when adaptation doesn’t hold. Ball socket inside the actuator wears or breaks; replacement is the only fix.
- Water ingress damaging the actuator electronics. 451-specific. Heavy rain, car wash, or driving through standing water can do it.
- Clutch worn out. Less common at this code than at the three-bar warning, but it does happen. Inspect clutch wear when actuator is out.
DIY check steps
- Read the codes. P0805 alone or paired with P2022, P202A, or three-bar warnings? Multiple codes points harder at the actuator.
- Check the battery. Anything below 12.4V resting, or that drops below 10V during cranking, replace before going deeper. Cheap and the right call regardless.
- Note the symptoms. Squeaky/scratchy noise when starting in gear + slipping on downshifts is the classic actuator-failing pattern.
- Inspect the actuator area for water staining if the symptoms came on after wet weather.
That’s about as far as DIY goes — you can’t adapt the actuator without a Smart-compatible scan tool, and you can’t replace it without one either (post-install coding required).
When to call a shop
This is a shop job for almost everyone. The actuator install requires MB Star (or Xentry, or community AutoFlash) for post-install coding. Some owners have replaced their own clutch and used a friend’s scan tool for adaptation; that’s the DIY path if you have access to the tool.
Important: the 451 clutch adjustment procedure that some online guides reference is only valid for 450 and 452 models — do not attempt to adjust a 451 clutch the way you would on a 450. Doing so can damage the actuator’s internal arm and ball socket, and you’ll be replacing the whole actuator instead of fixing it.
Aftermarket parts: SACHS is the OEM supplier and a viable alternative to dealer-priced parts. Genuine Mercedes is the safer bet on the actuator itself; eBay used actuators are a coin flip even with coding.
Plan on $1,000-1,500 at an independent Smart shop for actuator + clutch + coding. Dealer pricing runs higher.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Clutch actuator (genuine Mercedes recommended) | $400-900 part | Search Google |
| Clutch actuator + clutch combo install | $1,000-1,500 installed at indie shop | Search Google |
| SACHS clutch (OEM supplier) | $200-400 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Workshop / service manual PDF — full procedure with torque specs, hosted on smartcarmanuals.com.
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- SmartCarOfAmerica: Mileage on Replacing Clutch and Actuator (P0805 case)
- SmartManiacs: Broken clutch actuator (water ingress note)
- Evilution: Specific error codes (P2022 / P202A clutch system entries)
Stuck on this one?
SmartDiag-AI runs through the cheap-first checks with you, weighted to community-known patterns for your exact model. The link below pre-fills the code and model.