Navien furnace shows E051, heating demand not recognized or unit does not respond to thermostat

Safety: Turn off power before working on thermostat or low-voltage wiring.

• Verify thermostat type: Ensure the thermostat or Navien room controller is compatible with the furnace model and wired for the correct control type (e.g., dry contact, 24V).
• Check wiring at thermostat: Remove the thermostat from its base and inspect the wiring. Confirm that the wires are connected to the correct terminals as per Navien's wiring diagram. Tighten any loose connections.
• Check wiring at furnace: At the Navien control board, verify that the thermostat/room controller wires are landed on the correct low-voltage terminals and are secure.
• Test continuity: With power off, disconnect both ends of the thermostat cable and test continuity of each conductor with a multimeter. Replace the cable if any conductor is open or shorted.
• Bypass test (if allowed by manual): Temporarily install a jumper across the call-for-heat terminals at the furnace to see if the unit responds. If it does, the thermostat or its wiring is suspect. Replace the thermostat or room controller as needed.