What does Navien E021 Error Code - Causes & Fix mean?

E021 means the cold water inlet thermistor has an open circuit. Most common fix: replace the failed sensor or repair its wiring.

How do I fix Navien E021 Error Code - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with E021 active?

Generally no. Faults on Water heater units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for Navien E021 Error Code - Causes & Fix?

The most frequently replaced parts for Navien E021 Error Code - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is Navien E021 Error Code - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

Navien E021 Error Code - Causes & Fix

Navien E021 Error Code — What It Means

The E021 error code on a Navien tankless water heater indicates a cold water inlet thermistor open-circuit fault. The control board is not receiving a valid signal from the temperature sensor that monitors the cold water entering the unit. This usually means the thermistor itself has failed open or the wiring and connector between the sensor and the board are interrupted, corroded, or damaged.

Jump to Fix

Common Causes

Failed cold inlet thermistor The sensor itself has an open circuit and no longer returns a valid resistance reading to the control board.
Loose or disconnected wiring The harness connector at the thermistor or board has come loose, creating an intermittent or complete open circuit.
Corroded or damaged connector pins Water intrusion, corrosion, or physical damage at the connector prevents electrical continuity to the sensor.
Chafed or broken wire in the harness The thermistor wiring has been cut, pinched, or worn through somewhere between the sensor and the control board.
Control board input failure The thermistor and harness test good but the board input circuit is damaged and cannot read the sensor signal.

Step-by-Step Fix

Turn off power to the unit at the breaker and shut off the gas supply before opening the cabinet or touching any internal components.
Confirm the error code on the display panel and note whether it returns immediately after a brief reset or only under flow conditions.
Inspect the cold water inlet thermistor connector for looseness, corrosion, bent pins, water damage, or debris, and reseat the connector firmly.
Check thermistor resistance with a digital multimeter by disconnecting the sensor and measuring across its terminals, then compare the reading to your model’s service manual specification for that water temperature.
Test harness continuity from the thermistor connector all the way to the control board connector to locate any open wire or damaged section of the harness.
Replace the cold inlet thermistor if it reads open or out of specification, or repair and replace any damaged wiring or connectors if the sensor itself tests good.
Restore power and test the unit under normal flow conditions to verify the error does not return and the inlet temperature is being read correctly on the display.

Parts Often Needed

Part	Notes
Cold water inlet thermistor	Amazon \| Verify the exact part number for your Navien model from the service manual or contact Navien technical support with your serial number.
Thermistor wiring harness	Amazon \| Replace if the harness shows cuts, abrasion, or broken wires that cannot be reliably repaired with heat-shrink splices.
Main control board	Amazon \| Only required if the thermistor and all wiring test good but the board still cannot read the sensor input.

When to Call a Pro

E021 requires opening the unit, working with low-voltage wiring, and using a multimeter to diagnose open circuits. If you are not comfortable de-energizing the heater, accessing internal components, or interpreting resistance readings against manufacturer specifications, contact a Navien-certified technician. Navien technical support recommends that thermistor and sensor-related faults be handled by qualified service personnel who have access to model-specific service data and genuine replacement parts. Incorrect diagnosis or improper part replacement can leave the unit unable to regulate inlet temperature and may void your warranty.