SQ5 Defroster Failure, Common Fixes and Repair Guide

Had a similar problem with my 2018 Audi S5 last winter. With some experience working on Audi electronics, I can share what happened in my case. The window defroster and rear window heater completely failed. Initial diagnosis showed the thermal window system was getting no power. After checking the fuses, my workshop found the root cause was a severely discharged battery that was no longer holding proper charge. The weak battery was causing voltage drops, preventing the window demister system from functioning correctly. The repair involved: Complete battery test and diagnosis, Battery replacement, System reset, Testing all defroster functions Total cost was 290 Euro and took about 2 hours to complete. The heating system has worked flawlessly since then. Could you share: Current battery age? Any other electrical issues noticed? Are you seeing warning lights on the dash? Does this happen only when the engine is cold? This info would help determine if your issue matches what I experienced. Battery issues often affect multiple systems, not just the defroster repair components.

Thanks for the detailed insight. After full diagnosis at my last service (24834 KM), my battery tested fine but the window heater control module appears faulty. No other electrical problems or warning lights present, and the issue persists regardless of engine temperature. The workshop quoted 480 Euro for a new control module plus installation. Before proceeding, I wanted to confirm if anyone else found alternative solutions, since the defroster repair cost seems quite high for what appears to be a common thermal window system failure.

Thanks for the update. Interesting, my S5 actually ended up having the same control module issue after we initially thought it was the battery. When the replacement battery didnt fully resolve the window demister problems, further testing revealed a faulty heating control module. I faced the same dilemma with the high repair costs. The 480 Euro quote matches what I was given. Unfortunately, after researching alternatives, I found the heating coil control module needs to be an OEM part to work properly with the thermal window system. I tried a cheaper aftermarket module (320 Euro) but it caused communication errors with other systems. Had to switch back to OEM anyway, which ended up costing more in total than if I had gone with the original quote. For the rear window and windshield heating to work reliably, the control module really needs proper voltage regulation and system communication. The workshop installed the OEM module and reprogrammed the system, total fix time was about 3 hours. The window heater has worked perfectly since the repair. While expensive, I would recommend proceeding with the OEM module to avoid potential compatibility issues later. Have you noticed if the heating coil lines on the glass are getting any power at all? This could confirm if its purely a control module issue versus damaged heating elements.

Thank you for sharing those details. After checking both options, I decided to proceed with the OEM control module replacement. You were right about avoiding aftermarket parts, the window heater system is too integrated with other electronics to risk compatibility problems. The workshop confirmed no power was reaching the heating coils, supporting the diagnosis of a failed control module. The defroster repair has been scheduled for next week. They estimate 3-4 hours labor time since they need to access components behind the dash to replace and program the thermal window system module. Total cost will be 480 Euro as originally quoted. While not cheap, your experience with the aftermarket part convinced me its worth paying more now to avoid future heating issues. Will report back on how the repair goes for others who might face similar window demister problems in the future.