Kadjar temperature gauge and slow warm up

I had a similar issue with my 2017 Opel Insignia. Same symptoms, slow warm-up, CEL on, and weird temperature readings. In my case, it was indeed related to the active grille shutter system. The flap motor had failed, leaving the grille partially closed. Have you checked the fuse for the grille shutter motor? If present, that's the first thing to verify.

Thanks for the insight! I'll check the fuses. Where exactly is the fuse located for the grille shutter motor? Also, is there a way to manually check if the grille is opening and closing correctly?

On my Insignia, the fuse was in the engine compartment fuse box, but refer to your Kadjar's owner's manual for the exact location. Regarding manual inspection, with the engine cold, you should be able to see the shutters and, with some careful manipulation, feel if they move freely. However, for a definitive diagnosis, a workshop with proper diagnostic tools is recommended.

It turned out to be a defective fuse, just like you suggested! Replaced it for 52€ and the issue is gone. Thanks a lot for your help!