E05 displayed and cooling output does not respond to setpoint changes.

Safety: Isolate power to the HVAC system and thermostat at the breaker panel.

Steps:

• Remove the thermostat and inspect the cooling output terminals (often labeled COOL, Y, or similar) for signs of arcing, melted plastic, or loose screws.
• Verify wiring against the Schneider Electric installation diagram. Ensure that the cooling output is wired only to the control coil of a contactor or to a low-current input, not directly to a compressor or high-load device.
• Use a multimeter to check for continuity across the cooling relay when the thermostat calls for cooling (this may require a professional). If the relay never closes or is permanently closed, it is defective.
• If the output device (e.g., contactor coil) is shorted or drawing excessive current, replace that device first to avoid damaging a new thermostat.
• Replace the thermostat if the internal cooling relay/triac is confirmed faulty; these components are not typically field-replaceable.