Fault Code 16 - Water Sensor

[T350 Brad]

Just got my T350 back after having the ECU repaired, runs perfectly, but now when the temperature reaches about 91-92 degrees I'm getting Fault Code 16, Water Sensor. I know this is a common problem but the temps shown on the display aren't fluctuating and appear accurate, and the fans are working perfectly (cutting in sequentially at 92 and 94 degrees).

Strangely no alarm sounds when the warning comes up, and it stays 'til i've cancelled it.

Any ideas what this might be!?!? As I said, the car runs fine, no overheating, no coolant loss, and both sensors were replaced recently.

Any suggestions gratefully received before I approach my local Dealers.

Thanks
Bradders