Seat Altea Diesel Start Stop Problem

That's a tricky one. The start-stop system, sometimes called 'Eco mode' by manufacturers, relies on a healthy battery and a bunch of sensors. When you say error memory, do you mean an OBD2 scanner pulled a code? If so, what was the code? It's also worth checking if your battery is actually charging properly. A failing alternator can cause the battery to drain even while driving. Do your headlights dim at idle?

Yeah, the scanner showed an error related to low voltage. The headlights don't seem to dim, but how would I know for sure if the alternator is failing? Is there a way to test that without taking it to a shop?

Low voltage error plus start-stop issues on an older diesel points pretty strongly to the battery being the culprit. The start-stop system is very sensitive to battery health. Diesels, especially, need a strong battery to crank over. While you can test an alternator yourself with a multimeter, given the symptoms and the error code, a new battery is probably the best first step. Frankly, it's probably due for replacement anyway. The stalling could be the system trying (and failing) to restart the engine with insufficient power. I would suggest you go to the workshop.

Just wanted to say the shop confirmed it was the battery, like you said. New battery, idle stop is working again! Cost me about 290€. Thanks a lot for your help, I really appreciate it!