The short answer
It is almost always the battery: it is low, old, the wrong type (EFB or AGM), or new and not registered with the control unit. Before replacing anything, check that none of the conditions that switch it off apply (cold engine, climate control on full, seat belt undone) and read the fault codes with a diagnostic tool.
Start & Stop cuts the engine when you stop and restarts it when you release the brake or press the clutch. To do that without leaving you stranded, the car checks a long list of things before every stop. If anything doesn't add up, it leaves the engine running. So there are two very different situations: the system isn't acting because it shouldn't, or it isn't acting because of a fault.
When it is normal for the engine not to stop
Rule these out before you go looking for faults. In every one of them, Start & Stop stays inactive on purpose:
- The engine is still cold, or it is very cold or very hot outside.
- The climate control needs the engine: air conditioning on maximum or windscreen demisting.
- The battery is below the charge the car considers safe.
- Your seat belt is undone, or the driver's door or bonnet is open.
- The steering wheel is turned a long way, you are on a steep slope or you are in reverse.
- You have only just started the car, or haven't reached a certain speed since the last stop.
- The particulate filter is regenerating.
If you do short trips in winter, it is common for it not to work for weeks. That isn't a fault: the battery never gets enough charge.
The real causes, most common first
1. A tired or flat battery
This is cause number one. A car with Start & Stop works its battery hard, and once the battery loses capacity the system stops cutting the engine to protect it. It is often the first symptom, before the car even struggles to start.
2. A new battery that hasn't been registered
Many cars (BMW, Mercedes, the VAG group, Volvo, Ford and others) have a sensor that monitors the battery and a control unit that tracks its wear. If you replace the battery and don't tell the car, it still thinks the old one is fitted. The result: it charges badly and Start & Stop still won't work with a brand-new battery. It has to be registered with a diagnostic tool. We explain it step by step in how to register a new battery.
3. The wrong type of battery
Cars with Start & Stop use an EFB or AGM battery. If someone fitted a conventional one, it won't last and the system stops working. Check the label: if it doesn't say EFB or AGM, you have your first clue.
4. Battery sensor
The sensor sits on the negative terminal. If it fails or has a poor connection, the control unit can't trust the battery's condition and disables the system. It usually stores a fault code.
5. Door, bonnet, clutch or brake sensors
If the car thinks the bonnet is open or can't read the clutch pedal properly, it won't stop the engine. These also store a fault or show up in live data.
6. Starter motor or alternator
Less common, but an alternator that undercharges or a starter with problems will also make the car disable the system.
How to diagnose it with a scanner
- Full scan. Read the faults on every control unit, not just the engine. Battery sensor and charging faults are often stored in the energy management module or the gateway. We show you how in how to read and clear fault codes.
- Live data. Check the battery voltage and state of charge. Many makes have a value that tells you why Start & Stop is blocked at that moment. More in live data.
- If the battery is new, register it. Enter its capacity and type exactly as shown on the label.
- Charge the battery and test. After registering or charging, the system can take several journeys to start working again.
| What you see | Likely cause | What to do |
|---|---|---|
| No faults and a battery over 4 or 5 years old | Tired battery | Test it and replace it |
| New battery and it still doesn't work | Not registered | Register the battery |
| Battery sensor fault | Sensor or connection | Check the negative terminal |
| Bonnet, clutch or brake fault | Switch or sensor | Check it in live data |
Which tool you need
To read faults on every control unit and register the battery, an iCarsoft V2.0 for your make will do, such as the BMM V2.0 for BMW and Mini or the VAWS V2.0 for the VAG group. The V1.0 tools don't register batteries. If you have cars of several makes, the CR Pro and the CR Pro+ both register the battery. The CR Pro+ also has specific Start & Stop functions on cars that support them.
Don't disable it the crude way. Some people unplug the battery sensor or bridge the bonnet switch so the system never kicks in. That leaves stored faults and can make the alternator charge badly. If you don't like Start & Stop, use the button or, on some makes, change how it behaves through coding.
