Thermostat displays E5 and heating output is locked out; may occur after a short circuit or wiring change.

Safety first: Isolate power at the breaker. Verify that no voltage is present at the thermostat terminals.

Steps:

• Inspect load wiring: Remove the thermostat and check the heating output terminals (e.g., NO, COM, LOUT). Look for bare copper touching, crushed insulation, or signs of arcing. Correct any shorts and re-insulate as needed.
• Check load resistance: Disconnect the heating load wires and measure resistance across them. Compare to expected values from the heater specification. A near-zero reading may indicate a shorted heating element or actuator.
• Test thermostat output: With the load disconnected and power restored, see if E5 clears. If the error clears with no load but returns when the load is reconnected, the problem is in the external load or wiring.
• Internal failure: If E5 persists even with no load connected, the internal relay or triac is likely damaged. The thermostat must be replaced or professionally repaired.

Do not bypass: Never bypass the internal protection or bridge output terminals directly; this is a fire and shock hazard.