Tiida Auxiliary Heater Issue

Sounds familiar! I had a similar problem with my old Opel Astra, different brand but same symptoms with the parking heater. Did you check the glow plugs? They're often the culprit in these situations. They can cause all sorts of issues with the auxiliary heater.

Glow plugs, huh? I haven't checked those specifically. Where are they usually located, and are they easy to get to on the Tiida? Is there a way to test them without taking everything apart?

On my Astra they were a pain to get to, but I believe on your car, they shouldn't be too hard. Honestly, unless you're comfortable with a multimeter and some wiring diagrams, I'd just take it to a workshop. A defective glow plug was exactly my issue, cost around 440€ to get sorted including labour.

Thanks for the advice! Glow plugs make sense. I think I'll take it to a shop and have them diagnose it properly. Knowing what the problem likely is beforehand helps a lot. I will post the final cost when it's resolved!