Engine temp gauge craziness

[BHP231]

2012 Cayenne diesel, occasionally on cold start up the temp gauge needle goes to maximum and the temp light flashes.
Obviously the engine isn’t overheated, after a few minutes it settles to normal temperature.
There appears to be two sensors on coolant, one in the middle of the top of the engine, the other near the bottom of the radiator off to one side.
I changed the top one but this hasn’t fixed the problem, before I spend another $100 on a sensor, has anyone found this problem and got an idea on the fix?
Thanks.

[faverymi]

Check the coolant reservoir and it's level sensor

[Matt at Pelican Parts]

Are you certain it's a faulty sensor and not a another failing component causing the temp gauge to shoot up?

We do have a pretty handy R&R on the coolant tank but it looks like the sensor is simply a coolant level sensor and not connected to the gauge
https://www.pelicanparts.com/techarticles/Porsche_958/Coolant_Reservoir_Tank_Replacement/Coolant_Reservoir_Tank_Replacement.htm

[honerboys]

+1 ^^^on faverymi comment above - from what i understand if the car senses the coolant is low it immediately starts lighting up the dash with warnings - it happened to me also