Thermostat shows E31 and intermittently loses power or reboots when calling for heat or cool.

Safety first: Turn off power to the HVAC system at the breaker before opening panels or touching wiring.

• Inspect R and C connections: At the furnace control board, tighten the screws on the R and C terminals and ensure the wires are secure and not corroded.
• Check transformer rating: Locate the 24VAC transformer and read its VA rating (often 40VA). If multiple accessories are connected (humidifier, zone board, etc.), the transformer may be overloaded.
• Measure voltage under load (advanced): With a multimeter, measure AC voltage between R and C while the system is idle and while calling for heat or cool. A drop significantly below 24VAC under load indicates a weak transformer or wiring issue.
• Inspect thermostat cable: Look for signs of damage where the cable enters the furnace or wall. Replace the cable if insulation is cut or if continuity tests show intermittent opens.
• Replace transformer if needed: If the transformer output is low or unstable, have a qualified technician replace it with a properly sized unit.