You can adjust the temperature display up to 3 degrees in either direction (see page 9 of the manual) in the installation settings menu. Has anyone recently changed the installation settings?

A 5 degree difference in measured versus actual temperature is not normal. Have you had a power surge lately? I am thinking something has damaged the thermostat. Unfortunately the thermostat is not repairable. You will probably need to buy a new one.

Here is a link that might be useful: Honeywell 8600 manual

Thanks Mike_Home. We have not had a power surge lately. I am wondering if it's just age. The thermostat was installed 9 years ago.

If I have to buy another, can someone let me know what they think of the Honeywell Vision Pro 8000? It has a touchscreen--not sure if that's a good thing or not. That model is what my HVAC contractor recommended over the phone (I have an ongoing service contract with them, and they check my HVAC twice a year). Though they want to charge a lot more than it would cost me just to buy it myself online.

Is Honeywell making thermostats as good as they used to? Or should I be looking at another brand? It is important that I get 7-day programmability. TIA.

Here is a link that might be useful: Honeywell VisionPro 8000 Thermostat