Seat Arosa Diesel power loss and check engine light

Sounds familiar! I had a similar issue with my Arosa. Reduced engine power and the check engine light were the first signs. Have you checked the air flow? A faulty throttle body can mess with that and cause a response delay. Does the engine idling seem normal, or is it rough?

Air flow seems okay but I'm no expert. How would I properly test the throttle body itself?

Yeah, sounds like the throttle body. Unless you have diagnostic tools, it's tricky to be 100% sure. Could be a faulty sensor messing with the air/fuel mixture. Best bet is to get it hooked up to a code reader at a workshop to pinpoint the problem. Save yourself the headache of guessing.

Cheers for the help! Took it to a garage, they confirmed it was the throttle valve playing up. 265€ later, she's running like new. No more check engine light!