Astra preheating lamp stays on

Sounds like a pain. When you checked the connections, did you disconnect the battery first? Sometimes a full reset can clear things up. Also, have you tried reading the error code with an OBD reader? That would pinpoint the faulty component. If the connection looks okay and the reset doesn't work, it's probably a faulty heating module as you've already guessed.

Thanks for the quick reply! No, I didn't disconnect the battery - rookie mistake, I know. I don't have an OBD reader myself. Do you think it's something I could fix myself without any special tools or is it best to leave it to the pros?

Okay, not disconnecting the battery could cause some issues, so try that first! But if you're seeing smoke, and the check engine light is on, it is worth getting it scanned to confirm. Given the preheating unit is causing the issues, it's probably not worth attempting to fix yourself, since you mentioned the connections looking defective. It's better to take it to a workshop, and get it fixed with original spare parts instead of spending a lot of time checking every single part. You could probably get away with getting a mechanic with proper diagnostic tooling to replace the part and connections for you. This should solve all of your problems.

Thanks, I appreciate the advice. I went to my local garage, and they confirmed it was the preheating control unit, and they replaced the cable and plug, and replaced the defective part. Cost me 95€, but at least it's sorted now!