Thermostat Reading Wrong Temperature

Thermostat is mounted somewhere that doesn't reflect actual room temperature. Examples: exterior walls (cold/hot through wall), near vents (gets blasted with conditioned air), kitchens (cooking heat), bathrooms (shower humidity), direct sunlight (radiation heating).

Look at where the thermostat is mounted. Is it in a hallway away from rooms you actually occupy? Above an air vent? On an exterior wall? In direct sun any time of day?

Dust on the temperature sensor inside the thermostat insulates it and causes false readings. Common in homes with high dust levels or smokers.

Pull the thermostat cover off and look for dust on internal components. Or just gently blow it out with compressed air.

Over years, thermostat sensors slowly drift from accurate readings. Cheap mechanical thermostats drift the most.

Reading is consistently off by 2-5°F from independent thermometer. Worse than 5°F = real sensor problem, not just drift.

If the thermostat wire passes through an attic, garage, or exterior wall cavity, heat or cold from those spaces can conduct to the thermostat through the wire itself and affect readings.

Reading is off only in extreme weather (very hot or very cold). Pull thermostat off the wall and check if cold or warm air comes through the wire hole.

Sensor inside thermostat has actually failed. Readings are way off (10°F+), erratic, or won't change.

Try to set thermostat to different temperatures — does anything happen? Does reading make sense with weather? If completely disconnected from reality, the unit has failed.

Very old mercury-bulb thermostats need to be level to read correctly. If yours has been bumped or settled, readings will be off.

If you have an old round Honeywell or similar mechanical thermostat, hold a level against it. If it's not level, that's your problem.