Skoda Citigo parking heater malfunction

Sounds familiar. I had a similar issue with my Citigo's auxiliary heater a while back. Could not get it to start and the system threw an error. Did you check for any coolant leak around the heater unit? Sometimes a small leak can cause issues with the temperature sensor and prevent it from firing up. The auxiliary heater relies on the engine's coolant to provide heat, so if the coolant level is low or there is an issue with the flow, it might not work. Also, have you tried checking the blower motor for the climate control? A faulty blower motor can sometimes trigger errors related to the heating system.

Coolant leak, blower motor, temperature sensor… that's a lot to check. Where exactly is the temperature sensor located? And how would I check the blower motor? Is that something I can do myself, or should I just take it to a workshop?

The temperature sensor is usually located near the auxiliary heater unit itself, often inline with the coolant hoses. As for checking the blower motor, you'd need to access it, usually behind the glove compartment. You could try to see if it spins freely or if there's any obstruction. However, since you already have an error code and suspect the glow plug, it might be best to take it to a workshop. Replacing the glow plug on the auxiliary heater can be a bit tricky, and they can properly diagnose the system and rule out other potential causes like a faulty control unit.

Big thanks for the help. Turns out it was indeed the glow plug. 440€ later and the auxiliary heater is working again.