Thermostat reports 'No Power to R' or loses display power intermittently

Safety first: Turn off power to the furnace/air handler at the breaker before touching any low-voltage wiring.

Steps to troubleshoot:

• 1. Check breaker and furnace switch: Ensure the HVAC breaker and furnace service switch are ON.
• 2. Inspect thermostat wiring: Remove the thermostat from the wall plate and verify the R (or Rc/Rh) and C wires are firmly secured and not frayed.
• 3. Check furnace low-voltage fuse: Remove the furnace access panel (if you are comfortable and it is safe) and look for a small automotive-style fuse on the control board. Replace only with the same rating if blown.
• 4. Look for shorts: Inspect thermostat cable for damage where it enters the wall and furnace. Shorts between R and C or other terminals can blow the fuse.
• 5. Test transformer output: With power on and using a multimeter (only if you are trained), measure between R and C at the furnace control board. You should see ~24VAC. If not, the transformer may be failed.

When to call a professional: If fuses continue to blow or there is no 24VAC at the furnace board, contact an HVAC technician to diagnose and replace the transformer or repair wiring.