Thermostat display shows E1 and heating does not start or stops shortly after calling for heat.

Safety first: Turn off power to the thermostat at the breaker or by removing the thermostat from its powered backplate (if applicable). Verify power is off with a non-contact voltage tester before touching any wiring.

Steps to diagnose and fix:

• 1. Power-cycle the thermostat: Remove the thermostat from the wall/backplate for 30–60 seconds, then reattach. If E1 clears and does not return, it may have been a transient fault.
• 2. Inspect the room sensor area: On many Danfoss smart thermostats the room sensor is an integrated NTC on the main PCB behind the front cover. Remove the front cover carefully and look for obvious damage, cracked components, or contamination around the sensor area.
• 3. Check sensor connection: If the model uses a plug-in sensor module or a small 2‑wire harness to the NTC, ensure the connector is fully seated on the main PCB and that no pins are bent or corroded.
• 4. Measure sensor resistance: With power still OFF, disconnect the sensor from the board (if removable) and measure resistance with a multimeter. Compare to Danfoss service data (typical NTC values: ~10 kΩ at 25°C). An infinite reading indicates an open sensor.
• 5. Inspect PCB tracks: Look for cracked or burnt tracks between the sensor and the microcontroller. Lightly press the board to see if the fault is intermittent. If tracks are damaged, the main PCB usually must be replaced.
• 6. Replace faulty sensor or thermostat: If the NTC sensor is defective and not separately replaceable, replace the entire thermostat head unit. If it is a plug-in sensor, install a new Danfoss-compatible NTC sensor module.

When to call a professional: If you are not comfortable opening the thermostat or using a multimeter, or if PCB damage is visible, contact a qualified heating technician or electrician.