Smart Forfour Diesel Water Sensor Problems & Fixes

I had a very similar electrical fault on my 2007 Smart Fortwo diesel. Having some experience with car diagnostics, I first thought it was just a simple sensor issue, but it turned out to be more specific. The malfunction indicator showed similar symptoms, rough engine running and stored error codes. After taking it to a workshop, they found that the water sensor connection was loose due to a damaged cable harness. The auto electronics specialist identified that the plug connection to the sensor was the root cause, not the sensor itself. The repair involved fixing the wiring and properly securing the connection point. Total cost was 95€ for diagnostics and repair work. Since then, no more issues with the sensor or engine running. Before suggesting more specific solutions, could you share: Have you checked if the connector is visibly damaged? Are you getting any specific error codes? Did the issues start suddenly or develop gradually? Has the vehicle been exposed to heavy rain or water recently? This type of electrical fault is often connection-related rather than a failed sensor, but knowing these details would help determine if your case matches what I experienced.

Thanks for sharing your experience. My Forfour just had its service last week at 199032KM. Just checked the connector area you mentioned and noticed some corrosion around the sensor wiring. The sensor issue started gradually over the past month, getting worse after rainy days. Getting error code P0193 consistently now. The rough running is most noticeable during cold starts. Interesting that you found a wiring fix, much cheaper than replacing the entire sensor unit. Will have the electronics checked first before jumping to sensor replacement.

Thanks for those additional details. Your case sounds nearly identical to what I dealt with on my Smart, especially with the corrosion and rain correlation. The P0193 code combined with the corrosion you found strongly suggests its the same wiring issue I encountered. The gradual onset and weather sensitivity perfectly matches the pattern of a degrading connection rather than a complete sensor failure. Since your symptoms worsen after rain exposure, just like mine did, its highly likely moisture is entering through that corroded area and interfering with the sensor signals. The cold start issues occur because moisture accumulates overnight, causing poor connectivity until the engine compartment warms up. A couple points from my experience: Cleaning the corroded connections provided temporary relief, The permanent fix required replacing the damaged section of the wiring harness, Total repair time was about 2 hours for the wiring work, The malfunction indicator cleared within a few start cycles after repair Considering the similarities, I recommend having the wiring checked first. Much more cost-effective than needlessly replacing a potentially good sensor. The complete wiring repair should cost around 120€, significantly less than a new sensor unit. Make sure the repair shop thoroughly seals the connection points to prevent future moisture ingress and sensor testing issues.

Thanks for all the detailed info. This really helps confirm my suspicions about the wiring being the likely culprit rather than a failed sensor unit. The similarities between your experience and my situation, especially the rain correlation and cold start behavior, are quite striking. Will book the car in next week for a proper auto electronics inspection, focusing on the wiring harness repair you suggested. The 120€ estimate sounds much more reasonable than the 400€ I was quoted for a complete sensor replacement. Good to know about the temporary fix with cleaning too, might try that carefully this weekend until I can get it properly repaired. Will make sure they properly seal everything during the repair to avoid future electrical fault issues. Really appreciate the thorough explanation of the repair process and what to expect. This should save me both time and money by targeting the actual problem rather than just throwing parts at it.