Thermostat shows E09 and schedules or settings are lost after power cuts

Safety first: Isolate power before removing the thermostat from the wall. Do not attempt to solder or replace internal components.

• Check for removable batteries: Some thermostat variants include user-replaceable batteries. If present, replace them with fresh, high-quality batteries and reprogram the thermostat.
• Firmware reset: Perform a factory reset of the thermostat (note that this will erase all settings). Then reconfigure schedules and test whether they are retained after a controlled power cycle.
• Avoid abrupt power cuts: When possible, turn off the system using the fused spur switch rather than pulling fuses or breakers mid-operation.
• If settings still reset: The internal memory or backup circuit is defective. Replace the thermostat.