Smart Forfour Temperature Sensor Issues & AC Failure Fix

I had a similar issue with my 2003 Smart Fortwo CDI. With moderate experience working on these compact cars, I can share some insights about the HVAC system problems I encountered. The symptoms you describe match exactly what I experienced. The temperature gauge malfunctioned, showing erratic readings, and the cooling system seemed to have a mind of its own. After multiple attempts at sensor calibration, my workshop diagnosed a faulty air conditioning control unit rather than just a temperature sensor issue. The repair involved replacing the entire control unit, which also resolved the stored error codes. The total cost was 980€, including parts and labor. The heat exchanger and associated components were thoroughly checked during the repair to ensure no additional issues were present. To provide more specific guidance for your case, could you share: Current mileage, Any unusual sounds from the HVAC system, Whether the problems started gradually or suddenly, If you notice any temperature-related issues while driving, The specific error code stored in the system This information would help determine if your issue mirrors mine or might be something different in the cooling system.

Last service was completed at 154000km, about 2 months ago. I can confirm the problems started after a long highway drive. No unusual sounds from the HVAC system, but I do notice the engine cooling seems inconsistent. The temperature fluctuates more than usual while driving. The error code showing is P0128 which suggests coolant temperature is below the thermostat regulating temperature. I initially thought it was a coolant leak but didnt spot any obvious signs during vehicle maintenance checks. Will probably need professional diagnosis at this point, but good to hear your experience with a similar issue. Did you notice any warning lights on the dash before the full repair was needed?

Thanks for providing those details. Your symptoms are quite similar to what I experienced with my Smart Fortwo, especially the inconsistent engine cooling patterns you mentioned. Before the full repair, my temperature gauge readings became increasingly erratic. I did see the check engine light illuminate intermittently, particularly during longer drives. The sensor malfunction really showed itself during highway trips, much like your situation. The P0128 code you mentioned is telling. While my issue turned out to be the control unit, this error often indicates the cooling system isnt reaching proper operating temperature. In my case, the faulty control unit was misreading the temperature signals, causing similar symptoms. Before jumping to the full control unit replacement I had, you might want to have them check: Coolant level and circulation, Thermostat operation, Wiring connections to the temperature sensors, Heat exchanger condition These checks cost me about 120€ during diagnosis, but they were necessary to rule out simpler fixes. The irregular temperature fluctuations you describe match my experience precisely, and they worsened over time until the repair became unavoidable. Would you say the temperature variations are more noticeable during specific driving conditions?

Thanks for asking about warning lights. Yes, I did notice the check engine light coming on intermittently about a week before the major issues started. Actually had two warning lights, the engine management light and occasionally a temperature warning indicator would flash. Based on your advice, I will have the cooling system components checked professionally. The temperature variations seem most noticeable when climbing hills or during heavy acceleration. Also, the temperature gauge readings become more erratic after about 20 minutes of driving, which sounds similar to your experience with the control unit issue. Given the history you shared, I will have them inspect the control unit along with the temperature sensor and cooling system during the diagnosis. Better to check everything now than risk further issues. Will schedule an appointment this week since these temperature-related problems usually dont fix themselves. Do you recall if your Smart had any issues starting or with fuel consumption while these cooling system problems were occurring?