Seat Leon Diesel Won't Start Terminal Ignition Issue

Sounds like a frustrating issue. Terminal 15 is pretty critical. A defective relay switch or a corroded connection at the battery terminal could definitely cause a voltage drop and prevent the engine from starting. Have you checked the fuses related to the ignition system in the fuse box? Sometimes a blown fuse is the culprit. Also, use a multimeter to check voltage at the battery terminals and then at the starter motor while someone tries to start the car. That will give you a sense of where the voltage drop is happening, if there is one.

Where exactly is the relay switch for Terminal 15 located? Any diagrams available for the fuse box layout on a Leon diesel?

Finding the exact relay location without a specific diagram is tricky. Given the symptoms and error code, it's likely a relay switch issue or a break in the wiring. I'd suggest a workshop at this point. They have the proper diagnostic tools to pinpoint the fault and avoid further damage. It's probably a quick fix for them.

Thanks for the input. It turned out to be a defective relay switch in the fuse box. 95€ and the Leon is back on the road.