Thermostat displays E08 and loses power intermittently, rebooting when the system starts.

Safety first: Turn off power to the HVAC system at the breaker before working on wiring.

• Step 1 – Inspect R and C connections: Remove the thermostat and verify that both R and C wires are firmly clamped in their terminals on the Geo wall plate. Tighten any loose screws.
• Step 2 – Check transformer rating: At the furnace/air handler, locate the 24 VAC transformer and note its VA rating (e.g., 40 VA). If multiple accessories (humidifier, zone board, etc.) are connected, the transformer may be undersized.
• Step 3 – Measure voltage under load: With a multimeter, measure voltage between R and C at the thermostat while the system is running. If it drops significantly below 24 VAC (e.g., under 18 VAC), the transformer may be overloaded or failing.
• Step 4 – Remove extra loads: Disconnect any non‑essential accessories powered from the same 24 VAC circuit (e.g., add‑on Wi‑Fi modules, aftermarket relays) and see if E08 clears.
• Step 5 – Install a Geo‑compatible power extender or new transformer: If the transformer is undersized, have an HVAC technician install a higher‑VA transformer or a dedicated power extender kit for the thermostat.
• Step 6 – Replace thermostat if regulator is bad: If voltage is stable and sufficient but the thermostat still reboots and shows E08, the internal power regulator on the Geo PCB may be failing. Replace the thermostat head unit.