Thermostat shows generic 'ERR' on the display and does not follow programmed schedule.

Safety first: No internal wiring work is usually needed; avoid opening the unit unless instructed.

Steps to check:

• Soft reset: Use the reset button (if present) or remove power/batteries for 1–2 minutes, then restore. Many transient ERR states clear after a reboot.
• Check date/time: Verify that the date and time are set correctly. Incorrect time can cause schedule conflicts that some models report as ERR.
• Review installer settings: Enter the installer menu and check that all required parameters (system type, sensor mode, minimum on/off times) are set to valid values.
• Factory reset and reprogram: If ERR persists, perform a factory reset and re-enter your heating schedule from scratch.

Resolution: If the ERR message remains after a full reset and correct configuration, the internal logic may be faulty. Replace the thermostat.