Recently my Honeywell thermostat CT3500 was behaving erratically. It will switch on the heat if the temperature falls below the set temperature but will not switch off when it reaches the set temperature and exceeds it.

I called Honeywell and they advised me to reset the thermostat. I have disconnected, removed the batteries, etc but it will not reset. Everytime I put it back on it displays the set temperature. My Thermostat is about 4 yrs old.

Any solutions?