How the start-stop system works
With a manual gearbox the engine switches off when the car stops, the gearbox is in neutral and your foot is on the brake. It starts again when you press the clutch or release the brake, depending on the car. With an automatic the engine switches off when the car stops and starts when you release the brake or press the accelerator. Some newer cars switch the engine off while the car is still slowing down. Some cars also start the engine by themselves when the car in front moves off.
While the engine is off, the radio, lights and ventilation keep working, powered by the battery. That is why a car with start-stop has a stronger battery, EFB or AGM, and a sensor on the battery. Through the sensor the engine computer knows how charged the battery is and whether it may switch the engine off.
Start-stop doesn't switch the engine off: when it's normal and when the battery is to blame
Start-stop often doesn't switch the engine off even when you would expect it to. Before every stop the car's computer checks many things, so this is usually normal operation, not a fault.
Start-stop doesn't switch the engine off: it depends on the conditions
- The engine isn't warm, it is very cold or hot outside, the air conditioning is at full power: that is normal operation, not a fault.
- It hasn't switched the engine off for days and all of that is fine: it is a sign of a weak battery. The computer protects it from frequent engine starts, so it turns start-stop off.
When start-stop deliberately doesn't switch the engine off
The list differs a little from car to car, but on most cars the engine stays on in these situations:
- The engine has not warmed up yetThe engine has to warm up first, so that the heating in the cabin also works sooner.
- It is very cold or very hot outsideThe limit depends on the manufacturer. In winter, on many cars start-stop stops working already at around zero.
- The air conditioning or heating hasn't reached the temperature you setThe same applies when windscreen defrosting is on. In winter heated windows and seats, and in summer the air conditioning, use a lot of power.
- The battery is lowAfter short trips the battery doesn't get charged, so start-stop doesn't work until it is.
- The driver's seat belt is not fastened, or the driver's door or bonnet is openThe computer then assumes the driver is getting out of the car.
The steering wheel is turned a lot or the parking sensors are onWhile you are parking, the engine stays on.- The car is on an uphill or downhill slope, or towing a trailerOn a steep slope the engine stays on.
- The car has barely moved before stoppingAfter starting, the car has to cover a few metres and go faster than about 10 km/h.
- On a diesel, the particulate filter is being cleanedWhile the filter is being cleaned, the engine has to run. More about DPF regeneration
- The battery has just been replacedAfter a replacement the computer spends some time learning how to charge the new battery. If the new battery hasn't been registered in the computer, start-stop may not work at all.
- One of the computers in the car has stored a faultA fault in any part involved in start-stop turns it off.
Why the engine starts by itself while you are stopped
Start-stop starts the engine even without the accelerator or clutch if the battery drops below its limit, if many power-hungry devices are on, if the air conditioning can no longer hold the temperature or if the car starts rolling down a slope. This is normal operation too.
The battery for a start-stop system
A car with start-stop needs an EFB or AGM battery, the one the car maker specified.
A standard battery in a car with start-stop works for a while, then start-stop stops working properly and the battery wears out faster. We fit the type of battery your car needs and register it in the computer when the car requires it. More about car battery replacement
On many newer cars a new battery also has to be registered in the car's computer. More about choosing a car battery
Start-stop fault on the dashboard
When the car shows a start-stop fault on the dashboard, the computer has stored a fault. The cause can be the battery, the battery sensor or another part involved in start-stop. We read the fault with a diagnostic tool and tell you what is wrong. More about car diagnostics
How to turn start-stop off
You turn start-stop off with a button, on many cars next to the gear lever, often marked "Auto off". It stays off only until the end of that drive. The next time you start the car, start-stop is on again. If you turn it off while the engine is off at the lights, the engine starts straight away.
On some cars start-stop can be turned off permanently by coding the car's computer. On some newer cars this is no longer possible. More about car coding
Does start-stop harm the engine and the starter
A car with start-stop has a stronger starter, the part that starts the engine. It is built for several times more starts than a standard one. Engine makers also change the engine bearings, because an engine with start-stop moves off from standstill more often. Start-stop switches the engine off only once it is warm, so it doesn't add cold starts.
The battery in such a car works harder than in a car without start-stop. That is why it lasts less if it is the wrong type or if you only drive short distances, so the battery never gets fully charged.
How much fuel start-stop saves
According to the German motoring club ADAC, start-stop saves up to 15% of fuel in city driving, where the car often stops at traffic lights and in traffic jams.
