Thermostat displays E5 and system will not run or runs erratically

Safety: Turn off power to the furnace/air handler at the breaker. Use caution when measuring live low‑voltage circuits.

• Check C‑wire connection: Remove the thermostat from the wall plate. Verify that the C terminal has a firmly connected wire and that the same wire is connected to the C terminal at the furnace control board.
• Measure transformer voltage: With power on and using a multimeter, measure between R and C at the furnace board and at the thermostat wall plate. You should see about 24VAC. If voltage is low (<20VAC) or fluctuating, the transformer or control board may be failing.
• Inspect for shorts: Look for bare thermostat wires touching each other or the metal wall box. Repair any damaged insulation and ensure each conductor is fully inserted into its terminal.
• Try a different wall plate (if available): If you have another compatible Honeywell Home wall plate, test the thermostat on it to rule out a bad plate connection.
• Replace thermostat or call technician: If proper 24VAC is present and wiring is intact but E5 persists, the thermostat’s internal power circuitry is likely defective. Replace the thermostat or have a professional test the control board and transformer.