Nest thermostat shows E4 and reports that it is not receiving enough power from the HVAC system

Safety: This involves low‑voltage control wiring. Turn off power to the furnace/air handler and AC condenser at the breakers before touching any wires.

• Check wiring at Nest base: Remove the Nest display. Verify that the R (Rc/Rh) and C wires are fully inserted and the spring terminals clamp them securely. Tug gently to confirm they are locked.
• Inspect furnace control board: At the furnace/air handler, locate the control board. Confirm the same R and C wires are firmly attached to the R and C terminals. Tighten any loose screws.
• Add or repair C wire: If no C wire is present, run a new conductor from the furnace control board C terminal to the Nest C terminal, or repurpose an unused conductor in the thermostat cable as C (ensure both ends are connected to C).
• Measure voltage: With power restored, use a multimeter to check between R and C at the furnace board and at the Nest base. You should see about 24 VAC. If voltage is low (under ~20 VAC) or drops when the Nest calls for heat/cool, the transformer may be undersized or failing.
• Consider Nest Power Connector: If running a new C wire is not practical, install a Nest Power Connector at the furnace control board per the manufacturer’s instructions to provide stable power.