RS5 Cooling System Failure Due to Faulty Fan Control

I had a very similar issue with my 2019 S5 last summer. With some experience working on Audi cooling systems, I can share what happened in my case. The radiator fan behavior you describe matches exactly what I encountered. The erratic fan operation was caused by a faulty speed sensor, which prevented proper communication between the fan control module and the engine management system. The engine cooling was compromised because the radiator fan either stayed on constantly or failed to activate when needed. The workshop diagnosed it using specialized diagnostic equipment, and the repair involved replacing the speed sensor unit. Total cost was 250 Euro including parts and labor. The repair took about 2 hours, and the cooling system has worked flawlessly since then. Before you head to a workshop, it would be helpful to know: Are you noticing any unusual fan noise when it does run? Have you checked the coolant level? Are there any other warning lights besides the check engine light? Did you notice any fan blade damage or debris in the radiator area? This information would help determine if your issue is identical to what I experienced or if there might be other factors involved.

I really appreciate the detailed response. Just had my last service at 65000km about 2 months ago. The fan does make a slight whirring noise when running, and the coolant level is normal. No other warning lights are visible, and the radiator area looks clean with no visible damage to the fan blades. My mechanic suggested the thermal switch might be the culprit, but I am not convinced since the engine cooling issues persist even after cleaning the connections. This fan motor behavior seems very similar to your experience with the speed sensor. Would you recommend getting the speed sensor diagnosed first before exploring other potential causes? The quoted repair cost sounds reasonable compared to a full fan motor replacement.

Thanks for the additional details. Based on what you describe, your symptoms match my S5 issue even more closely now. That whirring noise from the fan motor was also present in my case, it was one of the telltale signs of the failing speed sensor affecting the fan motor operation. The reasonable coolant level and clean radiator area further support that the car overheating is likely related to the fan control system rather than a physical obstruction or coolant issue. While a faulty thermal switch can cause similar symptoms, the specific combination of constant running or complete non-operation of the fan points strongly toward the speed sensor. In my repair experience, starting with the speed sensor diagnosis makes sense because: It is less expensive than replacing the entire fan motor, The diagnostic process is straightforward with the right equipment, The repair time is significantly shorter, The symptoms match exactly Just be aware that prolonged car overheating can potentially cause additional issues. Until you can get it checked, try to avoid extended idling and monitor your temperature gauge closely. The temperature control issues should resolve completely once the speed sensor is replaced, just as they did in my case. The 250 Euro repair cost I mentioned included the diagnostic fee, which most shops will apply toward the repair if you proceed with their service.

Based on your helpful feedback and experience, I will schedule a diagnostic appointment this week focusing on the speed sensor. The similar symptoms and repair costs you shared give me confidence about the likely root cause. While my mechanic suggested the thermal switch, the evidence points more strongly toward the speed sensor, especially given the distinctive fan behavior and that whirring noise. Will keep the car idle time minimal until the repair and watch the engine temperature gauge carefully. Good to know the diagnostic fee typically applies to the final repair cost. The 250 Euro total seems much more reasonable than replacing the entire fan motor assembly. Thanks for helping narrow down the issue, this will save time and money at the workshop. Let me know if you experience any other car radiator issues down the road. This forum has been incredibly helpful for properly diagnosing cooling system problems.