Thermostat shows E08 and backlight or display intermittently shuts off; device may reboot randomly.

Safety: Turn off power to HVAC equipment before touching wiring. Use a multimeter only if you are comfortable and trained to do so.

• Step 1 – Check wiring at thermostat: Remove the thermostat and verify that R and C wires are firmly connected and not corroded or broken.
• Step 2 – Check wiring at equipment: At the furnace/air handler control board, confirm that the thermostat cable is secure on the R and C terminals and that the 24 VAC transformer connections are tight.
• Step 3 – Measure voltage: With power on, measure AC voltage between R and C at the thermostat. It should typically be 24–28 VAC. If significantly lower or fluctuating, the transformer or wiring may be failing.
• Step 4 – Reduce load: If multiple devices share the same 24 VAC transformer (zone panels, humidifiers, etc.), disconnect non‑essential loads temporarily to see if E08 clears.
• Step 5 – Replace transformer or thermostat: If voltage is low at the equipment, have a technician replace the 24 VAC control transformer. If voltage is stable but E08 persists, the thermostat’s internal power supply is likely faulty; replace the thermostat.