Audi S5 Overheating: CAN Bus Fault Dumps Coolant

Dealt with a very similar data link issue on my 2012 S4 last year. Have some experience with basic diagnostics and maintenance, but this one definitely needed professional help. The diagnostic trouble codes initially pointed to communication faults between multiple control units. Like your case, the temperature readings were all over the place, causing the cooling system to malfunction. Took it to a workshop specialized in VAG cars. They traced it to a corroded wiring harness connector near the ECU, the pins had deteriorated causing intermittent connection issues. The car diagnostics showed the control unit was getting conflicting signals, leading to cooling system pressure problems. The fix involved cleaning the connector pins and replacing the damaged section of the harness. Total cost was 95 Euro including diagnostic work. No issues since the repair. Before giving more specific advice, could you share: Current mileage? Any recent work done on the electrical system? Are there any other warning lights besides check engine? Does the overheating happen more in city driving or highway? These details would help determine if your issue matches what I experienced or points to something else.

Thank you for sharing your experience with the wiring issue. My current mileage is 147441 KM and the last service was at 140000 KM with the standard maintenance items. After your input, I checked and also noticed that the temperature readings fluctuate more during city driving. The control module seems especially confused in stop-and-go traffic. The only warning light is the check engine, no other indicators. I had no recent electrical work done, but this connectivity issue started shortly after driving through heavy rain, which might be relevant given your experience with corroded connectors.

Thanks for the additional details. The symptoms match even more closely with what I experienced, especially the timing after heavy rain exposure. That rainfall likely accelerated an existing corrosion problem in the wiring harness. The control unit behavior you describe in city traffic is particularly telling, my S4 showed the same pattern where the data link issues were most noticeable during low-speed driving. The communication fault would cause erratic temperature readings, triggering the cooling system to build excessive pressure. Given your mileage and the trigger event being rain exposure, Id strongly suspect water intrusion at one of the main harness connectors. The software glitch symptoms are likely secondary to this physical issue. Before taking it to a shop, you might want to: 1. Check the main harness connections at the ECU for signs of water damage 2. Look for green/white residue around connector pins 3. Monitor if the problem gets worse in wet conditions But based on the similarity to my case, this will need professional diagnostic equipment to properly trace the source of the connectivity issue. The good news is that if its the same root cause, its a relatively straightforward fix once properly diagnosed. Would recommend having it checked soon before the erratic cooling system operation causes any secondary damage.

Based on that conversation, I'll reply as (the original poster): Really appreciate the detailed follow-up. After your suggestion, I inspected the ECU area and found exactly what you described, there's visible corrosion residue on one of the main harness connectors. The timing with the rain and the matching symptoms in city driving certainly point to the same communication fault you experienced. Scheduled an appointment with a specialist shop for next week. They mentioned they have the car diagnostics equipment to properly test the data link connections and trace any control unit communication issues. The intermittent temperature readings definitely match the pattern you described with your vehicle. Will update once they identify the exact problem, but feeling much more confident now that we've isolated the likely cause. Makes sense that the control module would get confused with deteriorated connection points. Hopefully it will be a similarly straightforward fix like in your case.