Seat Toledo Diesel Water Sensor Issue

That sounds frustrating. The symptoms you describe, check engine light, error memory, and rough running, are quite indicative of a faulty sensor indeed. Given it's a diesel, the moisture sensor is particularly important. When you say 'cable or plug connection defective,' did you check the wiring harness for any visible damage? Frayed wires or corrosion could definitely cause intermittent issues.

I did a visual check of the wiring and plug, but didn't see anything obvious like frayed wires. Corrosion could be there, but it's hard to tell. I don't have a diagnostic tool myself. Is there a way to test the sensor or plug without one, or is it pretty much essential in this case?

It sounds like you've done some basic checks already. Without a diagnostic tool, testing the sensor becomes more difficult. You could try cleaning the contacts of the plug with electrical contact cleaner, but be very careful not to damage anything. Honestly, given the symptoms and the potential for misdiagnosis, it might be best to take it to a workshop. They'll be able to properly diagnose the sensor and rule out other potential causes. A faulty sensor can cause all sorts of issues, so it's worth getting it sorted properly. This is just my opinion, of course!

I got it sorted. The water sensor plug on my 2018 Seat Toledo diesel really was the culprit, just as you suspected. A new plug and a bit of cleaning, and the error is gone. The engine runs perfectly. 95€ well spent!