Navien E033 Error Code — What It Means
Error code E033 on a Navien tankless water heater indicates a Cold Water Inlet Thermistor 2 open or short fault. The control board has detected that the resistance signal from the secondary cold water temperature sensor is outside the valid operating range, meaning the sensor is either disconnected (open circuit) or internally shorted. This sensor is critical for the unit to calculate the temperature rise and modulate the gas valve correctly. Without a valid reading from Thermistor 2, the unit cannot safely heat water and will shut down.
The fault is usually triggered by a failed thermistor sensor itself, but can also result from loose or corroded wiring connections at the sensor terminals or the main PCB, or from a damaged wire harness. In rare cases, the main control board’s input circuit may be faulty, though the sensor and its connections are the primary suspects. The code may appear alongside E032, which covers similar cold water thermistor faults.
Before You Replace Anything
Some technicians replace the main control board first, assuming a board fault. Always test the thermistor resistance with a multimeter and inspect the wiring harness before ordering a new PCB, which is far more expensive than a sensor.
Common Causes
- Failed thermistor sensor (~55%) The internal sensing element of the cold water inlet thermistor has degraded or broken, causing an open or short circuit that the control board reads as invalid.
- Loose or corroded wiring connection (~25%) The wires connecting the thermistor to the main PCB may be loose, disconnected, or corroded at the terminal or mullex plug, interrupting the resistance signal.
- Damaged wiring harness (~12%) The wire harness leading to the thermistor may be pinched, cut, or damaged due to vibration or external stress, creating an open or intermittent connection.
- Control board failure (~8%) In rare cases, the main circuit board’s input circuit for the thermistor may be faulty, though this is less common than sensor or wiring issues.
Quick Diagnosis
Answer these to narrow it down fast.
Does the code clear after a 30-second power cycle?
No: The thermistor or its wiring has a permanent fault. Proceed to measure sensor resistance and inspect the harness.
Do you have a multimeter to measure resistance at the thermistor?
No: Call a technician to test the sensor and wiring. Replacing parts without confirming the fault can waste money on unnecessary components.
Are the wiring connections at the thermistor and PCB clean and tight?
No: Clean or reseat the connections and check for continuity in the wiring harness. A poor connection is often the cause.
Step-by-Step Fix
- Turn off power to the unit at the breaker or disconnect switch and wait 30 seconds, then power it back on to see if the code clears as a temporary reset.
- Locate the cold water inlet thermistor at the bottom of the unit near the cold water connection and verify the wiring harness and mullex plug are securely connected to the main PCB.
- Measure thermistor resistance using a multimeter at the sensor terminals. Compare the reading to the manufacturer’s expected range (typically 10kΩ to 50kΩ at room temperature, but consult your model’s service manual for exact values). If the resistance is infinite (open) or zero (short), the sensor has failed.
- Inspect the wiring harness from the thermistor to the control board for cuts, pinches, or corrosion. Check continuity with the multimeter if the sensor resistance is normal.
- Replace the faulty thermistor if confirmed failed. Order the correct cold water inlet thermistor 2 from Navien (part number varies by model) and install it, reconnecting the wiring harness to the PCB.
- Test board function if the sensor and wiring are good but the error persists. The main control board may need replacement, though this is rare.
- Clear the error code and run the unit through a full heating cycle to verify the fault is resolved and water temperature is stable.
Parts Often Needed
| Part | Notes |
|---|---|
| Navien Cold Water Inlet Thermistor 2 | Amazon | Part number varies by model. Contact Navien at 800-519-8794 with your serial number for the correct sensor. |
| Navien Main Control PCB Board | Amazon | Only if the sensor and wiring are confirmed functional but the fault persists. Rare cause. |
| Wiring Harness for Thermistor | Amazon | If the harness is pinched, cut, or corroded beyond repair. |
When to Call a Pro
Call a qualified technician for this repair. Navien tankless water heaters involve gas connections, electrical diagnostics on the control board, and precise sensor replacement. Misdiagnosing the fault can lead to unsafe operation or damage to the unit. A technician will have the proper multimeter, service manual resistance tables, and access to correct replacement parts through Navien. If your unit is under warranty, contact Navien Technical Support at 800-519-8794 before attempting any repair, as DIY work may void coverage. The technician can also verify that no other faults are present and that the gas valve modulation is working correctly after the sensor is replaced.
Rough cost: A pro service call runs about $200-400 including parts and labor.