Thread: 99 tempature reading

Just bought a 99 gt with the 3.8l, its my first grand prix, the car heats great but the temp gauge slowly creeps to maybe 140 degrees. the dealer said they replaced the thermostat. So am I looking at a coolant temp sensor failure in the block, or a bad gauge? Its reading way too low

also driver info center reports 14.5mpg average?? fuel economy seems normal for the car....