Safety: Disconnect power before working inside the unit.

Locate return thermistor: Find the thermistor attached to the return water pipe entering the Navien furnace. It may be clipped or threaded into a well.
Check physical mounting: Ensure the thermistor is firmly attached to the pipe or seated in its well so it can sense water temperature accurately. Re-secure any loose clips.
Inspect wiring: Follow the harness from the thermistor to the PCB. Look for pinched, cut, or burned wires. Repair or replace the harness if damaged.
Measure resistance: With the thermistor disconnected, measure resistance and compare to Navien's temperature-resistance chart. Replace the thermistor if readings are out of range.
Reset and test: After replacement, restore power, reset the furnace, and verify stable operation without E022. If the error persists with a known-good thermistor, the PCB may require replacement by a technician.

Navien furnace shows E022, unit may start but quickly shuts down

Possible Causes