Thermostat displays E5 and does not energize the heating/cooling output; may be accompanied by a hot or buzzing thermostat

Safety first: Immediately switch off the circuit breaker feeding the thermostat and the connected load. Do not continue to reset power until the fault is found.

Steps to troubleshoot:

• Remove the thermostat from the wall and inspect the wiring terminals for burnt insulation, melted plastic, or loose strands causing shorts.
• Verify that the connected load (e.g., electric floor heating mat, boiler control input) does not exceed the thermostat's rated current (commonly 3–16A depending on model).
• Disconnect the load wires from the thermostat output terminals.
• With the load disconnected, restore power and see if the E5 code clears. If it clears, the problem is likely in the load or field wiring.
• Insulation-test or continuity-test the load circuit with a multimeter or megohmmeter; repair any shorts or damaged cables.
• If E5 persists even with no load connected, the thermostat's relay or triac is likely damaged; replace the thermostat.

Note: Overcurrent faults can be dangerous. If you see charring or smell burning, have a licensed electrician inspect the circuit.