Thermostat shows E04 and does not energize the heating output relay.

Safety: Turn off power to the thermostat and the heating circuit at the main breaker. Verify with a non-contact voltage tester.

Steps:

• Remove the thermostat from the wall and visually inspect the back of the unit for signs of overheating near the heating output terminals (often labeled HEAT, W, or 1-2).
• Check that the connected load (e.g., electric underfloor heating mat, zone valve, contactor coil) does not exceed the maximum current rating specified by Schneider Electric for this thermostat model.
• If the connected load is too large, install an external contactor or relay module and use the thermostat to drive the contactor coil only.
• If you smell burnt electronics or see discoloration around the relay, the internal relay is likely damaged. The relay is usually not user-serviceable; the thermostat module should be replaced.
• After correcting any load issues, replace the thermostat with an identical Schneider Electric smart thermostat model and reconfigure settings.

Note: Continuing to use a thermostat with a damaged relay can cause overheating and fire risk. Do not bypass the relay or jam contacts closed.