Seat Leon Engine Performance Problems

Sounds like a tricky one. Reduced engine power and error codes definitely point to the engine control system. Have you checked the basics like battery voltage and the condition of the wiring harness? Sometimes a voltage drop can cause weird issues with the computer module.

I haven't checked those specifically. How would a low battery or wiring issue affect the engine control like that? And how would I go about checking those things myself?

A faulty main relay could definitely cause issues with the engine control. It might not be providing the computer module with consistent power. Given the symptoms and error codes, it's plausible. I'd still recommend taking it to a workshop for professional car diagnostics to confirm before replacing any parts.

You were spot on! I took it to a local mechanic. The car diagnostics confirmed a faulty main relay, which seems to have caused the engine control unit to act up. They replaced the relay, did a system reset, and now the engine performance is back to normal. The eventual repair cost was around 120€. Thanks a lot for your help!