Symptoms
- Thermostat display doesn't match a separate room thermometer
- AC seems to shut off too early or run too long based on this mismatch
- Comfort doesn't match what the thermostat claims
Likely causes
- CommonThermostat mounted near a heat source, vent, or direct sunlightLocalized heat or cold at the thermostat's exact location skews its reading.
- PossibleThermostat needs recalibrationSome thermostats drift slightly over time and have a calibration offset setting.
- PossibleThermostat mounted on an exterior wall with poor insulation behind itTemperature from outside can bleed through and affect the internal sensor.
- Less commonSensor failing or nearing end of lifeAn aging internal sensor can drift or fail to read accurately.
What to do
- Place an accurate separate thermometer next to the thermostat for at least 15 minutes and compare readings.
- Check that the thermostat isn't near a supply vent, lamp, sunny window, or drafty door.
- Check the thermostat's manual for a calibration or offset setting, and adjust if there's a consistent, small difference.
- If it's mounted on an exterior wall, consider whether relocating it to an interior wall would give a more representative reading.
- Replace batteries as part of general troubleshooting, since low power can occasionally affect sensor accuracy.
- If the reading stays significantly off after recalibration, the thermostat may need professional evaluation or replacement.
Tools and parts
- Separate room thermometer
When to call a pro
Call an HVAC technician if recalibration doesn't fix a significant, consistent temperature mismatch; the thermostat sensor may need replacement.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.