Ford: ford escort zx2, ford escort zx2, coolant temp

Question
just bought car few months ago temp gauge showed the car would warm up a little still not in middle, changed thermostat and now its stuck on 3/4 hot whether the car has cooled down or not.  So bottom line temp stuck whats wrong?  Thanks

Answer
First thing is make sure that the cooling system is full.. second, check the coolant temo sensor... (It is harder to do without a scan tool though) the easiest way to do it is to swap the coolant temp sensor with a known good one... try unplugging the coolant temp sensor and start the car... (the gauge should not move) try those out and let me know...