E09 appears and the thermostat reboots repeatedly or freezes; display may flicker.

Safety first: Isolate power at the breaker before opening the thermostat or junction box.

• Verify supply voltage: Using a multimeter, check that the thermostat supply (e.g. 230 V AC or 24 V AC depending on model) matches the rating on the Delta Dore thermostat label.
• Tighten terminals: Remove the thermostat from its base and tighten all power and control terminals. Loose connections can cause voltage drops and resets.
• Check for overload: Ensure that no high-current loads are connected directly through the thermostat if it is designed only for control signals. Use a contactor or Delta Dore receiver for high loads.
• Test with clean power: If possible, temporarily power the thermostat from a known stable circuit to rule out supply issues.

If E09 and rebooting continue, the internal power supply circuitry on the main PCB is likely damaged. Replace the thermostat or have the PCB replaced by a qualified technician.