Thermostat shows E04 and heating/cooling outputs are disabled.

Safety first: Turn off power to the thermostat and HVAC system at the breaker before removing the unit.

• Step 1 – Check ambient conditions: Ensure the thermostat is not installed above radiators, towel warmers, or in direct sunlight. If it is, relocate the thermostat to an interior wall away from heat sources.
• Step 2 – Inspect ventilation: Look for dust, paint, or wallpaper blocking the thermostat’s side or bottom vents. Gently clean with a soft brush or compressed air. Do not insert metal tools into vents.
• Step 3 – Power cycle: After cooling for at least 15 minutes with power off, restore power and see if E04 clears. If it returns quickly without obvious external heat, suspect an internal fault.
• Step 4 – Internal inspection (advanced): Remove the front cover and inspect the main PCB for discoloration, burnt components, or a relay/triac that appears cracked or melted. Overheated power components can falsely trigger overtemperature protection.
• Step 5 – Load check: Confirm that the connected load (e.g. electric underfloor heating mat) does not exceed the thermostat’s rated current. Oversized loads cause internal overheating. If the load is too high, install an external contactor/relay controlled by the thermostat.
• Step 6 – Replace thermostat: If the high‑limit sensor on the PCB is defective or the board shows heat damage, replace the thermostat assembly. Do not bypass or short any safety sensor.