I'm not 100% on this, someone way be able to confirm, but I've got a feeling there's two water temp senders.
In the picture below I've labelled them 1 and 2.
1. I think (not near car, going from memory) is a little metal stud, that a little lug slides over, this is the sender for the temp. gauge.
2. Is the water temp sender to the
ECU for cold start etc.
Like I said, I may be completely wrong, but just because your temp gauge is working, doesn't mean your
ECU temp sensor (2) is.