Nest thermostat displays E294 and reports that it cannot detect the R wire or that the R wire is misconfigured

Safety: Turn off power to the HVAC system before adjusting wiring.

• Verify R placement: Remove the Nest display and confirm the 24V hot wire is in the R terminal (Rc/Rh combined on many Nest models), not in another terminal.
• Re‑strip and reinsert: Cut back the R conductor to clean copper, strip 3–4 mm, and reinsert firmly into the R terminal until the clamp clicks.
• Check at furnace: Confirm the same conductor is attached to the R terminal on the furnace control board and that 24 VAC is present between R and C.
• Continuity test: With power off, test continuity of the R conductor between furnace and thermostat. Replace cable or use a spare conductor if open.
• Board output: If there is no 24 VAC at the furnace R terminal, the transformer or control board is likely faulty and needs professional replacement.