Symptoms
- Temperature reverts to a default setting soon after adjusting
- Schedule seems to reset periodically
- 'Hold' or 'Temp' indicator shows on the display
Likely causes
- CommonTemporary hold left activeA manual adjustment often defaults to a temporary hold that expires and reverts.
- CommonWeak or dying backup batteryBasic programmable thermostats often use a battery just to retain the schedule during power loss.
- PossibleSchedule set incorrectly, such as overlapping periodsA misconfigured schedule can look like it's not holding when it's actually just running as programmed.
- Less commonAging thermostat with a failing memory chipVery old units can develop memory issues that cause erratic behavior.
What to do
- Check the display for a Hold or Temp indicator and switch it to Schedule or Run mode if present.
- Replace the backup battery, typically a small coin cell or AA battery inside the unit.
- Review each scheduled period for overlaps or conflicting settings and correct them.
- Reprogram the schedule from scratch after clearing all existing periods, in case of a corrupted entry.
- If the schedule still won't hold with a fresh battery and clean programming, consider replacing the thermostat.
Tools and parts
- Replacement battery
- Thermostat manual
Thermostat backup batterySearch: “programmable thermostat backup battery”
When to call a pro
Consider replacing the thermostat if it's several years old and continues to lose its schedule after a battery change and reprogramming.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.