Why has my start-stop system stopped shutting the engine off?
Short answer: Start-stop systems only activate when many conditions are met, including battery state of charge, engine and cabin temperature, and accessory loads, so a weak battery or extreme weather commonly disables it.
The system checks a long list of conditions before stopping the engine: battery health and charge, coolant temperature, outside temperature, climate control demand, brake vacuum, steering angle and more. If any condition is not met, the engine stays running, which is normal behavior.
The most common cause of a system that never activates is the battery. An aging battery, a battery that was not registered after replacement, or a fault with the battery monitoring sensor will often disable start-stop to protect starting ability.
Test the battery, check the battery monitoring sensor data on a scan tool, and confirm registration was done if the battery was replaced. Check for stored codes as well. Drive conditions also matter: short trips in cold weather may never allow it to activate. Explain the operating conditions to the customer before replacing parts.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.