VW Up Defroster System Fails, Both Windows Affected

Had a similar heating issue with my 2017 VW Polo last winter. As someone with moderate experience working on VWs, I initially thought the rear window heater and front defrost problems were related to the heating coils or wiring. Turned out the car defrost system was failing because the battery was severely discharged. The workshop diagnosed it quickly, the low voltage was preventing the heating elements from getting enough power to function properly. This also explained why the error codes appeared when activating the window heater. Total repair cost was 290€ including: Battery testing and diagnosis, New battery installation, System check of all heating elements, Final testing of defrost functions The whole repair took about 2 hours. Since fixing the battery, both the front and rear window heating have worked perfectly. What battery warning signs have you noticed? Does your car have trouble starting, or show any other electrical issues? Knowing the battery age and whether you have other electrical symptoms would help determine if your case is similar. Also worth checking if your alternator is charging properly, as this can contribute to battery drain and affect the heating system performance.

Thanks for the detailed response. I got my last service done at 45000km, and the rear window heater was working fine then. No battery issues at startup or other electrical problems so far. The auto defrost stopped working quite suddenly, one morning both front and back just failed. The thermal window system was working perfectly the day before. I checked the fuses already but they seem fine. When activating the heating coil now, I can see the indicator light come on briefly before it shuts off with the error code. Makes me think its not battery-related in my case since everything else electrical works normally. I will have the workshop check the heating elements and wiring to see if theres a short somewhere in the system. Hope its not as expensive as a full replacement of the window heater components.

Thanks for those additional details about your thermal window problem. That sudden failure pattern is quite different from what I experienced with my VW when the battery was the culprit. In my case, the rear defroster and front heating showed gradual decline before completely failing. The behavior you describe, with the indicator light briefly illuminating then shutting off, actually points more toward a defective control module or a break in the heating element circuit. After dealing with my own window demister issues, I learned these systems typically fail in one of two ways: Gradual decline (usually power-related), Sudden complete failure (usually component-related) A workshop should be able to test the heating elements with an ohmmeter to check for breaks in the circuit. If theres a break in the rear window heating grid, repair costs typically run 350-450€ depending on whether the entire glass needs replacement. The control module is another common failure point in these cars, that repair usually costs around 200-300€ if thats the source of your heating issue. Would suggest having them start with testing the heating elements and control module since your battery and electrical system seem to be functioning normally.

Thanks for helping narrow this down. You are right that my sudden failure pattern seems different from a battery-related issue. Had not considered the control module could be the problem. The car defrost system still shows the same behavior, brief light then shutdown with error code. Called the workshop and booked it in for next week. They will start by testing the heating elements and rear window components as suggested. The estimate they gave was 380€ if its just the control module or up to 500€ if the entire rear defroster grid needs replacement. At least that gives me a realistic budget range to work with. Good point about having them check the complete circuit with an ohmmeter. Will make sure they test both the front and rear window heating elements thoroughly to find any breaks. Really hoping its just the control module and not the full heating grid that needs replacing. Thanks again for sharing your experience. The workshop visit should reveal which part of the window demister system is actually causing the problem.