Yaris Start Stop System Failure

Start-stop issues can be tricky. You're right, the battery is often the culprit, especially with that mileage. The start-stop system relies heavily on a healthy battery for a quick restart. Is the starter motor sounding weak when the engine restarts, or does it seem normal? A weak starter could also point to battery issues or a failing starter motor. Does the warning light come on only when the engine shuts off unexpectedly, or does it appear at other times too?

The starter motor sounds normal during restarts, with no signs of weakness. The warning light comes on whenever the engine shuts off unexpectedly and stays on until I restart. Does your assessment of a battery issue remain unchanged? Or are there other factors I should consider?

Since the starter sounds normal and the light is consistent with the shutdowns, it sounds like the start-stop system is being disabled due to a voltage drop. These systems are super sensitive! While it could be something else, a weak battery is the most common reason, especially on a Yaris of that year. I'd recommend getting the battery tested. It’s likely on its way out. If the battery checks out, then you might have to dig deeper, but I'd start there. Probably best to visit a workshop for the battery check anyway.

I had the battery tested at a workshop, and it was heavily discharged and defective, as suspected. Replacing it fixed the issue. The total repair cost was 290€, including parts and labor. Thanks for your help.