Vanette Temperature Gauge Issues

I had similar issues with my old Opel Astra, although it wasn't a diesel. The temperature gauge was all over the place, and the engine light was on constantly. In my case, the thermal management system was malfunctioning due to restricted airflow to the radiator grille. It messes with the engine cooling, which can cause all sorts of weird readings and the slow warm-up you're describing. Have you checked for any obstructions blocking the grille itself?

Thanks for the tip! I'll definitely check the grille for obstructions. But where exactly should I be looking? Is it just a visual inspection for leaves and debris, or is there something more technical I should be checking for?

Start with a visual check for anything obvious blocking the airflow – leaves, plastic bags, anything like that. But honestly, given the symptoms, it might be worth having a workshop take a look. In my case, it was a faulty sensor causing the whole problem. They charged me around 60€ including the sensor.

Took it to the workshop, and you were right! Turns out the cooling grille was indeed restricted, causing all the temperature issues. They replaced a defective fuse, and it cost me 52€. Thanks for pointing me in the right direction!