VW Lupo Defrost System Failure, Heating Not Working

I had a very similar issue with my 2003 VW Polo, which shares many components with the Lupo. Based on some experience working on VWs, I can say electrical problems like this are fairly common in these models. The rear defroster and window heater completely stopped working in my case. After attempting basic troubleshooting, I took it to my regular workshop. They diagnosed a severely discharged battery that was damaging the electrical system. The low voltage was preventing the heating elements from getting enough power to function properly. The fix included: Battery replacement, Testing all heating circuits, Checking rear window heater connections, Resetting error codes Total cost was 290€ and the car defrost systems have worked flawlessly since then. Before recommending specific checks for your Lupo, could you share: Battery age, Whether dash warning lights are active, If other electrical components are acting strangely, Current battery voltage if you have a multimeter This would help determine if youre dealing with the same root cause or potentially a different heating issue.

Last serviced in November at 141631km and I wanted to add some details to the discussion. After checking the fuses as suggested, I found the window demister fuse was completely blown. Replacing it temporarily fixed the front defroster but the rear window heater remained dead. The new fuse blew again within a day. Used a multimeter and found the voltage was fluctuating between 11.2V and 12.8V, which seems low. The battery is only 2 years old but the alternator might be the culprit here since other electrical components started acting up too, dashboard lights occasionally dim and the radio cuts out. Think I will need to test the alternator before addressing the window heater circuits. The defroster repair might be more complex than initially thought.

Thanks for those additional details about your Lupo, this actually mirrors exactly what happened with my Polo. The symptoms you describe strongly suggest an alternator issue, just like I initially suspected with my car before the battery diagnosis. While my first repair focused on the battery, I actually had to go back because the problems returned. The alternator was indeed the root cause, causing the same voltage fluctuations you described. The failing alternator was: Unable to maintain steady voltage for the window demister system, Causing repeated fuse failures in the thermal window circuits, Making the rear window heating cut out intermittently, Affecting other electrical components The complete fix required: New alternator installation, Replacement of damaged window heating circuits, New fuses, Full electrical system check Total cost was 520€, significantly more than the initial battery-only repair, but it permanently resolved all heating issues and electrical problems. Before replacing any more fuses, get the alternator tested first. The fluctuating voltage will keep damaging components and blowing fuses until the root cause is fixed.

Thanks for the input. After getting the alternator tested today, it confirmed what you suggested, its putting out inconsistent voltage. Makes total sense why the rear defroster and other electrical components were acting up. Got a quote of 480€ for alternator replacement including labor. Going to have them check the window demister circuits and thermal window components while they have it apart since the rear window heating might have been damaged from the voltage issues. Will post an update once the repairs are done to confirm if this fixes both the alternator and the defroster repair problems. Hopefully others with similar Lupo heating system issues can benefit from this thread. Really appreciate all the detailed responses and troubleshooting tips. Saved me from replacing more blown fuses before finding the actual cause.