Safety: Turn off power before accessing internal wiring.

Locate the thermistor: The supply water thermistor is typically clipped or threaded into the supply outlet pipe of the Navien furnace. Refer to the service manual diagram for exact location.
Inspect wiring and connector: Check the thermistor harness for cuts, abrasions, or signs of overheating. Ensure the connector at the thermistor and at the main PCB is fully seated and free of corrosion.
Test the thermistor: Disconnect the thermistor and measure its resistance with a multimeter. Compare the reading to the Navien resistance vs. temperature chart. If the reading is far off from expected at room temperature, the thermistor is defective.
Replace if necessary: If faulty, remove the old thermistor (unscrew or unclip) and install a new Navien OEM thermistor with any required thermal compound. Reconnect the harness securely.
Reset and verify: Restore power, reset the unit, and confirm that the displayed water temperature is reasonable and that E021 is cleared.

Navien furnace displays E021, unit will not start heating cycle

Possible Causes