Thermostat display completely dead, no backlight or response to button presses.

Safety: Work only on the thermostat itself; do not open the boiler.

Steps to restore power:

• Replace batteries: Install new, correct‑type batteries, ensuring proper polarity. Test again.
• Check contacts: Inspect battery terminals for corrosion or bent contacts. Clean gently and bend contacts slightly to improve pressure if needed.
• Verify mounting: If the thermostat clips onto a backplate, remove and re‑seat it to ensure good contact with low‑voltage pins.
• Test supply (wired): For wired units, have a technician verify that the low‑voltage supply from the boiler or interface is present and correctly wired.

If the thermostat remains dead with known‑good batteries and proper mounting, the internal PCB is likely faulty and the unit should be replaced.