Thermostat displays E5 and cannot control the heating actuator or boiler relay.

Safety first: Isolate mains power to the thermostat and connected heating equipment. Do not attempt relay or triac replacement without proper electrical training.

Steps to diagnose and fix:

• 1. Confirm load rating: Check the connected actuator(s), pump, or boiler control input against the thermostat's maximum output rating (see Danfoss manual). Disconnect any loads that exceed the rating.
• 2. Visual inspection of PCB: Remove the thermostat from the wall and inspect the output relay/triac area for burn marks, cracked components, or discoloured PCB. If visible damage is present, the main PCB must be replaced.
• 3. Test output function: With power restored and under safe conditions, command the thermostat to call for heat and listen for relay click. If no click and E5 persists, the relay drive circuit is likely faulty.
• 4. Check wiring to actuators: Ensure that no actuator wires are shorted to neutral or earth, and that polarity is correct for 24V or 230V systems as specified by Danfoss.
• 5. Replace thermostat or control module: In most Danfoss smart thermostats, the output relay/triac is not field-serviceable. Replace the thermostat head unit or the associated wiring centre / control module that contains the faulty output.

When to call a professional: Any work involving mains voltage or PCB-level repair should be carried out by a qualified electrician or heating engineer.