My model 1F86-344 is two years old. If I set the house temp to 74 it will run the a/c continuously even after the temp shows the desired setpoint on the digital display. I can only turn off the a/c by manually raising the thermostat to 75 degrees. It has worked perfectly for two years but started this odd behavior two nights ago. I did hear a strange hissing sound from the thermostat a day before the problem started.