SMAX water sensor malfunction

I encountered a similar issue with my previous Mercedes C-Class, though it was a different sensor. The symptoms you describe are consistent with what I experienced. It could indeed be the sensor itself, but I would suggest thoroughly inspecting the wiring and connector first. Auto electronics can be finicky, and a seemingly insignificant electrical fault can cause such problems. Have you checked for any corrosion or damage to the wires leading to the sensor?

Thanks for the tip! Corrosion is a good shout, I'll check that. When you say 'thoroughly inspecting', what exactly did you do with your C-Class? Did you test the sensor somehow, or just visually inspect the wires?

In my case, a visual inspection was insufficient. I used a multimeter to check the continuity and voltage at the connector, according to the official Mercedes-Benz repair manual. Based on my experience, if you are unsure about testing the sensor yourself, it may be prudent to take the vehicle to a qualified workshop. The repair on my C-Class was similar in nature and relatively inexpensive.

Okay, thanks for the advice. I'll give the wiring a proper look with a multimeter, but if I'm still stuck, I'll take it to a workshop. It was a defective cable/plug connection, just like you suspected. Cost me 95€ to get it sorted!