Audi Q5 Defrost Issues: Faulty Heating Coil Alert

Having worked on cars as a hobby for a few years, I faced a similar heating issue with my 2006 Q7 last winter. The auto defrost system was acting up, showing error codes and barely warming the windshield. After getting it diagnosed, the root cause was a heavily discharged battery causing electrical system issues, which affected the window demister and overall heating performance. The thermal window system needs proper voltage to function correctly. The repair involved: Full battery diagnostic test, Battery replacement, Electrical system reset, Testing all defroster functions Total cost was 290 Euro including parts and labor. The heating system worked flawlessly after the fix. To better assist with your specific case, could you share: Current battery age, Any other electrical issues noticed, Whether the problem occurs when engine is cold or warm, If the issue is constant or intermittent, Current voltage reading if you have access to a multimeter Sometimes these heating issues can be misleading, as they often appear as defroster repair problems but stem from basic electrical system faults.

Thanks for sharing your experience. My latest service was done at 175000 KM, about two months ago. The rear defroster seems to be the main culprit based on your similar situation. I checked the voltage on the battery, its showing 12.1V when idle which seems a bit low. I noticed the problem is worse during cold starts, and the rear window takes much longer than usual to clear up through the auto defrost system. Will get the battery tested before diving into more complex heating system repairs. Useful tip about checking the electrical system first.

Good catch on checking the battery voltage. 12.1V at idle definitely indicates a weak battery, very similar to what I encountered. Since you mentioned the slow clearing during cold starts, this strengthens the electrical system theory. The heating coil system in these models requires consistent 13.8-14.2V to function properly. With your current reading, the window demister and auto defrost features cant receive enough power to heat effectively. A quick test you can do: run the engine at about 2000 RPM and check if the thermal window system performs better. If it does, this confirms the charging system is still good and its mainly a battery issue. In my case, after the battery replacement: Cold start rear window defrosting time dropped from 15 minutes to 3 minutes, Error codes cleared automatically, Window heating worked consistently regardless of engine temperature Based on the mileage you mentioned, if the battery hasnt been replaced recently, its likely due for replacement anyway. Suggest getting both a battery and alternator test before other repairs.

Thanks for the detailed explanation of battery voltage impacts. The test at 2000 RPM does show slightly better performance for the rear window heating, just as you described. The defrost time improved a bit at higher RPMs, but still not as quick as it should be. Your experience really helps narrow down the issue. Since my battery is actually 4 years old now that I think about it, getting it replaced seems like the logical first step before looking into more complex rear defroster repairs. The car maintenance record shows the original battery lasted about 5 years, so this one is definitely approaching end of life. Will get both the battery and alternator checked to rule out any charging system problems. Really helpful to know the proper voltage range should be 13.8-14.2V for the heating coil system to work correctly.