Thermostat displays E5 and may indicate 'High voltage' or 'Power surge detected'; system may lock out temporarily.

Critical safety warning: High voltage miswiring can cause fire or electric shock. If you suspect 120/240V on low-voltage wiring, stop and call a licensed electrician or HVAC technician immediately.

Basic checks (for experienced DIYers only):

• Turn off all power: Shut off breakers to the air handler/furnace and outdoor unit.
• Inspect wiring at thermostat: Confirm that only low-voltage (small gauge, typically 18 AWG) wires are connected to the thermostat. No Romex or large-gauge line-voltage wires should be present.
• Check transformer connections: At the indoor unit, verify the primary (120/240V) and secondary (24V) sides of the transformer are wired correctly and not cross-connected.
• Look for burn damage: Inspect the thermostat and control board for scorch marks or melted insulation.
• Power back on and monitor: If no miswiring is found and the E5 clears after a power cycle, a transient surge may have occurred. Consider adding a surge protector designed for HVAC systems.
• If E5 recurs or damage is visible: Do not continue operating the system. Have a professional test the transformer, control board, and thermostat and replace any damaged components.