E09 displayed and thermostat reports power supply error; screen may flicker or reboot intermittently.

Safety: Turn off power to the HVAC system at the breaker before touching thermostat wiring.

Steps:

• Remove the thermostat and check that the R (or +24V) and C (common) wires are firmly secured in their terminals and not corroded.
• With power restored and using a multimeter (only if you are trained), measure AC voltage between R and C at the thermostat. It should typically be between 20–30 VAC. If it is lower, the HVAC transformer or wiring may be undersized or failing.
• If voltage is low at the thermostat but correct at the furnace/air handler, inspect the cable run for damage or loose connections at junction points.
• If voltage is correct but the thermostat still shows E09 and reboots, the internal power supply circuit may be defective; replace the thermostat.
• Do not connect the thermostat to a transformer that also powers heavy loads beyond its rating; use a dedicated 24 VAC control transformer if necessary.