Thermostat display shows E1 and heating does not start even when room temperature is below setpoint.

Safety first: Turn off power to the thermostat at the boiler isolation switch or circuit breaker before opening any housing. Do not work on live 230V circuits.

• Power cycle: Remove the thermostat from its wall plate or remove batteries (for battery models like Eurotronic Spirit/Comet), wait 2 minutes, then restore power. Check if E1 clears.
• Inspect sensor connection: For wall-mounted Eurotronic smart thermostats, gently remove the front cover. Locate the small NTC sensor leads (usually two thin wires going to the front of the PCB). Ensure the plug is fully seated and there is no visible corrosion.
• Check for damage: Look for crushed, cut, or pinched sensor wires, especially where the cable enters the housing. If damaged, the sensor or harness must be replaced.
• Measure resistance (advanced): With power off and sensor disconnected, measure resistance across the NTC with a multimeter. At ~25°C, most Eurotronic NTCs are around 10kΩ. Infinite (open) or near 0Ω (short) indicates a failed sensor.
• Replace sensor: If readings are out of range or E1 persists, replace the NTC room sensor with a compatible Eurotronic sensor assembly. Reassemble, restore power, and reconfigure if required.