VW Phaeton Window Heating System Failure Fix Guide

I had a very similar issue with my 2009 VW Passat diesel. As someone with moderate experience working on VWs, I first noticed the auto defrost system failing completely, then both front and rear window heating stopped working. After some basic checks, my car showed the same symptoms, the window demister was not functioning and multiple electrical faults appeared. The workshop diagnosed it as a heavily discharged battery that was causing electrical system issues. The old battery was not maintaining proper voltage, which affected all auxiliary systems including the heating elements in the windows. The complete fix included: Battery replacement, Full electrical system diagnostic, Reset of all control modules, Testing of all heating elements Total cost was 290€ and the car defrost systems have worked perfectly since. The rear window heating and all other electrical components returned to normal operation immediately after the repair. To help diagnose your specific case, could you share: Has the battery been replaced recently? Are you experiencing any other electrical issues? Does the problem occur more frequently when the engine is cold? Which specific error codes are showing up? This information would help determine if your Phaeton is facing the same underlying issue as my Passat.

Just had it serviced 2 weeks ago and the mileage reads 66971km. After checking with my documentation, the battery was replaced during the last winter service. Your suggested battery-related fix unfortunately does not match my symptoms. The thermal window issues persist even with the new battery and proper voltage readings. The window heater malfunction shows consistent error patterns regardless of engine temperature or operating conditions. The defroster repair history shows no previous incidents of this type. The error codes specifically point to the heating elements circuit rather than a power supply problem. Currently getting quotes from several workshops since both front and rear window heating systems require specialized diagnostic equipment for the Phaeton electrical system. Would appreciate if anyone has experience with actual heating element or control module replacements in these models.

Thanks for sharing those details about the previous battery service and error patterns. After reading about your Phaeton symptoms, I now suspect my initial Passat diagnosis focused too narrowly on the battery. My follow-up experience might be relevant here: About 3 months after the battery replacement failed to solve the problem, I discovered the real culprit was a faulty control module specifically for the window heating systems. The auto defrost function would trigger but no actual heating occurred in either the front or rear glass. The repair involved: Control module replacement (180€), Heating circuit verification (90€), System recalibration (120€) This completely resolved both the front defroster and rear window heating issues. The error codes cleared and never returned. The specialized diagnostic equipment revealed microscopic breaks in the heating elements themselves, which the control module was detecting. Your consistent error patterns match this scenario much closer than my initial battery theory. Since both cars use similar heating systems, I recommend having the workshop specifically check the heating element control module and circuit continuity. The specialized equipment you mentioned will be crucial for accurate diagnosis.

Thanks for the detailed follow-up regarding the control module replacement. The error codes from my Phaeton definitely align more with your second repair experience than the battery solution. After consulting with the main workshop today, they confirmed the defroster repair will require checking both the control module and the thermal window elements. The consistent error patterns are indeed pointing to a circuit issue rather than a power supply problem. Their initial quote includes: Full diagnostic scan: 120€, Control module testing: 90€, Potential module replacement: 210€, System calibration: 140€ The window demister symptoms match exactly what you described, the system triggers but no actual heating occurs in any of the windows. Going to proceed with the full diagnostic next week and will update once they identify whether its the control module or the heating elements themselves causing the malfunction. Really appreciate the insight about the specialized testing equipment being crucial for proper diagnosis. This helps confirm Im taking the right approach with a properly equipped workshop rather than attempting simpler fixes first.