Thermostat displays E5 and screen may be dim or unresponsive

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breaker before touching thermostat or furnace wiring. Low‑voltage circuits can still short and damage components.

Steps to diagnose:

• Check thermostat seating: Ensure the thermostat is fully snapped onto the wall plate. Poor contact between the thermostat pins and wall plate can cause intermittent power.
• Inspect R and C connections: Remove the thermostat from the wall plate. Verify that the R (or Rc/Rh) and C wires are firmly clamped in their terminals with no loose copper strands. Tug gently to confirm they are secure.
• Check furnace control board fuse: At the furnace/air handler, remove the access panel (power off). Locate the low‑voltage 3–5 A blade fuse on the control board. If blown, replace with the same rating and inspect for shorts in thermostat wiring.
• Measure voltage (if you have a multimeter): With power restored and access panel in place (safety switch engaged), measure AC voltage between R and C at the furnace board and at the thermostat wall plate. You should see approximately 24 VAC. If present at the furnace but not at the thermostat, there is a wiring break.
• Check for damaged cable: Inspect visible thermostat cable for staples through the jacket, crushed sections, or splices that may have come loose.

When to replace or call a pro: If proper 24 VAC is present at the thermostat wall plate and E5 persists, the internal power supply circuit of the thermostat may be defective. Replacement of the thermostat or professional diagnosis is recommended.