Nest thermostat shows E8 and may shut down heating or cooling to protect the system

Critical safety: Immediately turn off power to the HVAC system at the breaker if you suspect a short circuit. Do not repeatedly reset breakers without finding the cause.

• Inspect Nest base wiring: Remove the Nest display and visually inspect all wires. Ensure no bare copper is exposed beyond 3–4 mm and that no two conductors can touch.
• Remove unauthorized jumpers: Nest thermostats do not require manual jumpers between Rc/Rh or other terminals. Remove any jumpers that were carried over from an old thermostat.
• Check cable along its path: Where accessible, inspect the thermostat cable for crushed sections, nail or screw penetrations, or melted insulation. Repair with proper splices or replace the cable if damaged.
• Inspect furnace control board area: At the furnace/air handler, verify that low-voltage wires are not pinched under the access panel or touching sharp metal edges.
• Replace blown fuse if present: If the low-voltage fuse on the control board is blown, replace it with the same rating only after correcting the short.

If the E8 error returns after these steps, a professional should perform continuity and insulation resistance tests on the wiring and components.