I have similar problem:
upon cold start gauge on dash goes to full red in 1 or 2 seconds.
When disconnecting single wire coolant sensor, and starting car gauge on dash stays at zero.
However, when replacing with new sensor, gauge on dash goes to full red at startup, very surprised by this.
(not possible the car is red hot yet at cold start)
This is the new sensor:
[image|1506993]
Wondering what else can check for, seems this coolant temp sensor is at the end of the line of things. There is nothing else feeding into that dash gauge right?