Smart Fortwo's Hidden Control Unit Causing Coolant Crisis

As someone with moderate experience working on cars, I encountered a very similar issue with a Smart Forfour (2015, gasoline). The symptoms were nearly identical, particularly the control unit communication fault and cooling system problems. After some initial troubleshooting, my workshop discovered it was related to a wiring issue affecting the coolant temperature sensor connection. The software glitch in the engine control unit was actually a secondary problem caused by faulty signal transmission from the sensor. The repair involved: Complete car diagnostics scan, Testing the wiring harness, Replacing a corroded connector, Recalibrating the control unit Total cost was 95€ for parts and labor. The fix resolved all three symptoms: the overheating, check engine light, and coolant pressure problems. To provide more specific guidance for your case, could you share: What diagnostic trouble codes are showing? Have you noticed any correlation between the overheating and specific driving conditions? Has the coolant been recently changed or system pressure tested? Are there any other electrical issues present? These details would help determine if your issue stems from the same root cause as mine.

My latest service was done at 74100km, where they performed routine maintenance and found no major issues. The control unit communication fault started showing up only after that. Based on your experience, I checked the coolant temperature sensor connector and noticed some green corrosion. The CAN bus diagnostics showed error code P0115 and P0128, both related to the engine coolant temperature circuit. The overheating mostly occurs during city driving when the engine is under varying loads. I had the coolant changed during the service, but no pressure test was done. The connectivity issue with the control unit seems intermittent, which matches your description of a wiring problem. Given the similar symptoms and relatively low repair cost in your case, I will have the workshop focus on checking the wiring harness and connectors first. Has anyone else experienced these specific diagnostic trouble codes with similar symptoms?

Thanks for those additional details about the error codes. Your symptoms match exactly what I experienced with my Smart Fortwo. The P0115 and P0128 codes were the same ones I got, and the corrosion on the temperature sensor connector is a telltale sign. The intermittent control unit communication problems combined with these data link issues are classic indicators of corroded connections affecting the sensor signals. In my case, the workshop found that moisture had gotten into the wiring harness near the coolant temperature sensor, causing both the corrosion and unreliable readings. One thing to note: if the control unit receives incorrect temperature data, it cant properly regulate the cooling system, leading to the pressure buildup you described. The car diagnostics initially pointed to a broader communication fault in my case too, but it was really just poor sensor data causing the control unit to malfunction. The good news is that fixing the wiring issue and cleaning or replacing the corroded connector should resolve all three problems. In my case, the repair was straightforward and the issues havent returned in over 8 months of driving. Before taking it to the workshop, try disconnecting and reconnecting the battery. This sometimes helps reset the control unit communication and could provide temporary relief while you schedule the repair.

Based on everyone's feedback, I feel more confident about the repair direction now. Those CAN bus diagnostics showing P0115 and P0128 codes match up perfectly with the symptoms others described. The green corrosion I found on the coolant temperature sensor connector seems to be the smoking gun, especially given how the control module communication errors are intermittent. I tried the battery disconnect suggestion and it did temporarily improve things, but the diagnostic trouble codes came back after a few hours of city driving. The wiring issue definitely seems to be causing incorrect temperature readings, which explains both the overheating and pressure problems. I'll schedule an appointment to have the wiring harness inspected and the corroded connections repaired. The 95€ repair cost mentioned is much more reasonable than I expected, especially considering replacing major components. I appreciate everyone sharing their repair experiences, this helped narrow down the problem and saved me from potentially expensive unnecessary repairs. Would still be interested in hearing if others have dealt with similar control unit communication faults in their vehicles.