Audi Q5: Fatal Mix of Cooling and Control Unit Failures

Having dealt with a similar issue on my 2014 Audi Q3 2.0T, I can share my repair experience from a DIY enthusiast perspective with moderate technical knowledge. The symptoms matched yours exactly, overheating, coolant pressure issues, and CAN bus communication problems. The root cause turned out to be corroded wiring connections at the engine coolant temperature sensor, causing both the cooling system malfunction and control unit communication errors. Initially, I suspected separate issues, but the workshop found that moisture had damaged the wiring harness connecting the coolant temperature sensor to the engine control unit. This created a data link failure, causing incorrect temperature readings and cooling system pressure regulation problems. The fix involved: Replacing damaged connector pins, Installing new wiring between sensor and control unit, Cleaning corroded terminals, Updating control unit software Total repair cost was 95 Euro, mainly for parts and programming. Before suggesting specific solutions for your case, could you share: Current coolant temperature readings, Which specific control units show communication faults, Whether issues started gradually or suddenly, If any recent cooling system work was performed This information would help determine if your Q5 is experiencing the same connectivity issue I encountered.

Last service was at 75806 KM about 2 weeks ago. After reading through my scan data more carefully, I found the software glitch causing temperature misreadings appears during cold starts only. The control unit seems to lose connection with the temperature sensor for about 30 seconds after startup. The workshop initially suggested replacing the entire cooling system, but based on your experiences, I will have them check the wiring connections and data link integrity first. The communication fault codes specifically point to the engine control module losing signal from multiple sensors. My temperature gauge now fluctuates wildly between normal and hot within minutes, which matches what you described regarding faulty sensor readings. The connectivity issue seems worse on cold mornings, and the sensor terminal connectors do look quite corroded when inspected closely.

The cold start symptoms you describe definitely match my experience. After dealing with this on my Q3, those temperature fluctuations during startup are a classic sign of corroded sensor connections affecting the control module readings. A key observation from my repair: the control unit communication fault became more frequent as moisture exposure continued. If your terminal corrosion is visible, the internal wiring damage is likely already affecting the data link between sensors and the engine control module. Quick tip I learned while fixing mine, the coolant temperature sensor wiring often deteriorates first near the connector housing where moisture tends to collect. When checking connections, pay special attention to: Wire insulation condition near connector points, Terminal pin surface corrosion, Connector housing seal integrity The scan showing multiple sensor communication losses strongly suggests a shared wiring issue rather than individual sensor failures. Getting the connectivity issue resolved should normalize both the temperature readings and cooling system pressure. I would definitely push back on complete cooling system replacement. My repair proved that addressing the control module communication fault fixed both the sensor readings and pressure regulation problems. Let us know what the wiring inspection reveals. Based on the similarities to my case, you might be looking at a similar 100 Euro range repair rather than a major cooling system overhaul.

Thanks for your reply and detailed repair experience. Your diagnostic insights about checking the wiring definitely helped narrow down my issue. I took the car in today and had the technician inspect the sensor connections first. True to your suggestion, they found significant corrosion at the temperature sensor wiring harness. The control module was getting inconsistent signals due to damaged wiring, which explained both the communication fault codes and erratic cooling system behavior. The final repair matched your case almost exactly: Replaced corroded wiring section, Installed new connector assembly, Updated control unit software to resolve lingering communication faults, Total cost came to 110 Euro The temperature readings are now stable even during cold starts, and the cooling system pressure has returned to normal. No more check engine light or control module errors. Really glad I asked here before authorizing a complete cooling system replacement. This definitely confirms these seemingly separate issues can stem from the same wiring-related problem. The CAN bus communication appears fully restored now that the sensor is getting proper connectivity to the engine control module.