Short summary
The start-stop system shuts off the engine of a stationary vehicle when suitable conditions are met and restarts it before moving off; however, even if the driver leaves the system switched on, it may not engage at every stop. The engine control unit evaluates a large amount of data before stopping the engine.
Engine schedule
The main factors affecting the system’s operation:
- The battery’s charge level
- Engine temperature
- Air conditioning demand
- Outside temperature
- The vehicle’s electricity consumption
If the engine has not reached its operating temperature, the system may not engage, particularly on short-distance drives. In some models, the seat belt, door and hood status, the way the brake pedal is pressed, the gear position, and turning the steering wheel too far also play a role.
New EFB steps
Since the conditions vary by make and model, the system sometimes not working is not, on its own, considered a fault.
The most common reason for deactivation is the battery’s state of charge; even if the engine starts on the first crank, the battery may not meet the start-stop requirements. These vehicles generally use EFB or AGM batteries; a suitable product should be selected when replacing the battery, and in some models the new battery must be registered with the system.
Technical details
While the air conditioning is trying to cool the cabin, during heavy heating in winter, or when defogging, the engine may be kept running; in hybrids with electric compressors, the system may behave differently. For the battery, the state of health, charge level, and charging system should be evaluated together rather than the voltage.