These instructions apply to the Smart Fortwo Mk2 (W451) 2007-2015.
For other models, please choose your vehicle here.
The Smart Fortwo (W451, 2007–2015) in mhd (micro-hybrid drive) form uses a Start/Stop system that shuts the engine off at a standstill to save fuel. If you’d rather keep it running — in traffic, when manoeuvring, or before working under the bonnet — you can switch it off with the dash button. The system is a normal economy feature, not a fault, and its warning light simply reports whether it’s active.
How Start/Stop Works on the mhd Fortwo
The mhd system uses a belt-driven starter-generator to stop and restart the little three-cylinder engine quickly and smoothly. Come to rest and the engine shuts down; lift off the brake or dip the clutch and it fires straight back. It only stops the engine once a set of conditions is met — mainly a well-charged 12 V battery, a warmed-up engine, and a cabin that’s already at temperature. On a car used for lots of short hops the battery often stays too low for the system to allow a stop.
Before You Start
- Do this with the engine running, or the ignition fully on, so the button and light respond.
- Only mhd micro-hybrid W451s have the system — if there’s no ECO/mhd button, your car doesn’t have it.
- If you’re about to open the bonnet, switch the system off first so the engine can’t auto-restart.
- Remember the setting only lasts for the current drive — there’s no permanent-off menu.
Turning Start/Stop Off
- Press the Start/Stop (ECO/mhd) button on the dashboard.
- The Start/Stop warning light changes state to show the system is now switched off.
It Resets Every Time You Start
Start/Stop re-enables automatically each time you start the engine. There’s no permanent off setting — press the button again after each start if you want it disabled.
If the System Won’t Stop the Engine Anyway
Even with Start/Stop active, the engine won’t cut unless conditions are met:
- The 12 V battery is sufficiently charged — short trips leave it too low.
- The engine is warm enough.
- The car is fully stopped.
- The cabin climate demand is satisfied — on a cold or hot day it may keep running.
A Start/Stop that rarely works on these cars is very often a tired battery.
What to Expect After
Once the warning light changes state, the engine will idle normally at every stop for the rest of that drive. Next time you start the car the system arms itself again, so press the button once more if you want it off. Nothing is stored or reset in the process — it’s a simple per-trip toggle.
Frequently Asked Questions
How do I turn off Start/Stop on a Smart Fortwo W451?
Press the Start/Stop (mhd) button on the dash. The warning light changes state to confirm the system is off, and the engine will stay running at stops for the rest of that drive. It re-enables when you next start the engine.
Does it stay off permanently?
No — the W451 has no permanent-off setting. Start/Stop re-enables every time you start the engine, so the only way to keep it off is to press the button once at the start of each drive.
Why doesn’t the engine stop at lights?
Usually the 12 V battery isn’t charged enough, or the engine isn’t warm yet. An unsatisfied cabin climate demand or the car not being fully stopped will also hold it running. On these cars a battery that’s past its best is the most common reason.
Do all W451s have Start/Stop?
No — only the mhd micro-hybrid versions. Non-mhd cars don’t have the button.
Why does mine almost never stop the engine?
On the W451 mhd this is nearly always a tired 12 V battery that can’t reach the charge the system requires. The battery is small and works hard, so it’s a common wear item; having it load-tested is the first thing to check if Start/Stop has effectively stopped working.
Is it safe to keep using Start/Stop every day?
Yes — the mhd hardware, including the belt-driven starter-generator, is designed for constant cycling. Everyday use is fine. If the stop-and-restart feels rough or hesitant, that usually points to the battery rather than the system itself, so get the battery checked.
If a Start/Stop fault light stays on, look up the code on autodtcs.com.
Independent guide — not affiliated with Smart. Always follow your official service manual and safety precautions. Read our full disclaimer.