Thermostat shows E8 and the display may dim or reboot intermittently, especially when calling for heat.

Safety first: Turn off power at the breaker before accessing thermostat wiring. Low-voltage circuits can still be hazardous if miswired to mains.

Steps to diagnose and fix:

• 1. Verify supply voltage: With a multimeter and power ON (only if you are competent), measure the voltage at the thermostat power terminals. Compare to Danfoss specifications (e.g., 24V AC or 230V AC). If voltage drops significantly when heating is called, the supply is inadequate.
• 2. Tighten terminals: With power OFF, check that all power and neutral/return conductors are firmly clamped and that no copper is exposed outside the terminals.
• 3. Check transformer or power source: For 24V systems, confirm the VA rating of the transformer is sufficient for the thermostat plus all connected actuators. Replace with a higher-rated transformer if undersized.
• 4. Remove excess load: Ensure that high-current devices (pumps, valves) are not powered directly through the thermostat if it is only rated for control signals. Use a Danfoss wiring centre or external relay module instead.
• 5. Replace thermostat if internal PSU faulty: If supply voltage is stable but E8 persists and the thermostat reboots, the internal power supply on the main PCB may be defective. Replace the thermostat head unit.

When to call a professional: Any measurement on live circuits should be done by a qualified electrician or heating engineer.