Toyota iQ Preheating Lamp Issue

I had a very similar experience with my iQ, though I can't recall the exact mileage. The symptoms you describe definitely ring a bell. It sounds like your diagnostic hunch about the glow control unit and the potential cable/plug issue is reasonable. Did you perform any diagnostic tests using an OBD reader? That could help to verify what is going on with your car.

Yes, I did a diagnostic test, and it indicated a fault within the glow plug control circuit. The error code pointed to an open circuit or a short to ground within the control unit. I've also inspected the wiring harness visually but haven't found any obvious signs of damage. Was your fault code similar? Was the car stalling?

The fault code I retrieved also indicated an issue with the glow plug control circuit, very much aligning with what you're seeing. I didn't experience stalling, fortunately. Given that you've already done a diagnostic test and visually inspected the wiring, it may be time to consult a qualified mechanic to further diagnose the issue. Replacing the unit is not a big deal, but you need to find the root cause of the problem before you do it. From my own experience, and considering your symptoms and diagnostic results, it really does sound like the glow control unit is the most probable cause. I'd suggest going to the workshop.

Based on your recommendation, I visited a trusted local workshop. As you rightly suspected, the glow control unit was indeed the culprit. The technician confirmed a defective plug connection. The total repair cost, including parts and labor, amounted to approximately 95€. Everything works perfectly now!