Nest thermostat displays E5 and reports a power or R wire issue, often after calling for heating or cooling

Safety: Turn off power to the furnace/air handler and outdoor unit at the breakers before inspecting wiring or fuses.

• Inspect Nest base wiring: Remove the Nest display and verify the R (Rc/Rh) wire is fully inserted and clamped. Straighten and re‑strip the wire if necessary so that 3–4 mm of bare copper is exposed and fully under the clamp.
• Check furnace control board: Open the blower compartment and locate the control board. Confirm the R wire from the thermostat cable is firmly attached to the R terminal.
• Inspect low‑voltage fuse: Look for a small automotive‑style blade fuse (commonly 3A or 5A) on the control board. If it appears blown (melted link), replace it with the same rating. Do not upsize the fuse.
• Check for shorts: Inspect thermostat cable along its path for crushed, pinched, or exposed conductors that could short R to C or another terminal. Repair or replace damaged cable.
• Test operation: Restore power and reattach the Nest display. If E5 returns immediately, there may be an intermittent short or a failing transformer; a technician should test the system further.