Thermostat powers on but does not heat the room; heater never turns on even when setpoint is higher than room temperature.

Safety first: Turn OFF the breaker and verify power is off before touching wiring.

Steps to check:

• Listen for switching: When raising the setpoint, listen closely for a faint click or change in relay noise from the thermostat (if applicable). No sound may indicate a failed output stage.
• Inspect LOAD wiring: Remove the thermostat and ensure the LOAD wires are firmly secured and not broken.
• Check heater continuity: With power OFF, measure resistance across the heater terminals. An infinite reading indicates a failed heater.
• Measure voltage to heater: If qualified, with power ON and a heat call active, measure voltage at the heater. No voltage with a call for heat suggests a failed thermostat output.

If the heater and wiring are good but the thermostat never energizes the LOAD, the output triac/relay is likely defective. Replace the thermostat.