E05 displayed and thermostat will not turn on heating or cooling; internal safety lockout active

Danger: This fault may involve 24V control circuits and potentially higher voltages in the HVAC equipment. Turn off power to the furnace/air handler and outdoor unit at the breakers before proceeding.

• Step 1 – Visual wiring check:
  • Remove the thermostat from the wall plate.
  • Inspect the R, C, Y, W, G, O/B, and other terminals for loose strands of wire touching adjacent terminals.
  • Ensure only one conductor is under each screw and that insulation is intact up to the clamp.
• Step 2 – Isolate the thermostat:
  • Label and disconnect all thermostat wires from the wall plate.
  • Restore power to the HVAC system with the thermostat disconnected.
  • If the HVAC system powers normally (no breaker trips), the short is likely in the thermostat or its relays.
• Step 3 – Test with minimal connections:
  • Turn power off again.
  • Reconnect only R and C to the thermostat and power it up.
  • If E05 appears immediately with no call for heat/cool, the internal relay or driver is likely damaged.
• Step 4 – Inspect HVAC wiring (if E05 only appears on call):
  • If E05 appears only when calling for heat (W) or cool (Y), inspect those circuits at the furnace/air handler for shorts to ground or other terminals.
  • Repair any damaged thermostat cable or miswiring.
• Step 5 – Replace thermostat if relay is shorted:
  • Internal relay or triac failures are not field‑repairable.
  • Replace the thermostat (or main control PCB if available as a part) if E05 persists with known‑good wiring.