Did you try putting in a fresh set of batteries? If the batteries are weak what may be happening is when the thermostat is on, the voltage on batteries drop which could cause a temperature reading error. When the thermostat is off, the voltage rises and the temperature reads correctly.

In any case this is an 11 year old thermostat that sells for $42. Not exactly a precision instrument built for years of service. If the new batteries doesn't solve the problem then consider getting a new thermostat.

I replaced the batteries, and it has not changed anything. This thermostat has 7-wires going to the unit. How do I know which is the right thermostat to buy and how to wire it correctly?

Here is the Simple Comfort 2200 manual. Is is a 2 stage heat and cool for heat pumps thermostat. You need to buy a thermostat compatible with heat pumps and supports 2 stages of heating and cooling.

Each wire is connected to a terminal with a letter (G, C, Y1, etc.) The letter determines the function of the wire. You should take a photo of the wiring before disconnecting anything and make a list of the wire color and it's letter designation. The wire color may or may not coincide with the letter (ex: G connected to green, R connected to red). But if they do then it makes it easier to keep track of everything.

Honeywell has several DIY thermostats. You can read the manuals on line to verify the thermostat is correct. The is also an 800 support number you can call for help.

I assume there is a hole behind the thermostat where the wires come through. Is the hole insulated? that is, is there some fiberglass or something to block air from comming through?