Need help troubleshooting thermostat error
 

Hey Guys,
New M3 owner here. When I took the car in for the pre-sale inspection, the DME threw (105) 69 Engine Temperature Signal Implausible (1x; Not
Present). I was told by the tech there that the thermostat needs to be replaced.
I took possession of the car today and even after driving 30 miles on the freeway, the temperature gauge needle stayed pegged in the middle. No sign of over-heating, is that possible with a stat failure:hmm:
What do you guys think could be the problem? Any ideas on troubleshooting this?