Navien E039 Error Code — What It Means
The E039 error code on a Navien tankless water heater signals an abnormal water flow sensor fault. The unit’s flow sensor, which detects water movement through the system, is either defective or not sending a valid signal to the control board. Without a working flow sensor, the heater cannot confirm that water is flowing and will not initiate the ignition sequence to heat water.
This is classified as a part failure rather than a user-serviceable condition. The sensor itself may have internal damage, the wiring harness connecting it to the control board may be loose or corroded, or in rare cases the control board may be failing to read the sensor signal. Navien recommends that only a licensed HVAC or gas fitter diagnose and repair this fault.
Before You Replace Anything
Homeowners sometimes replace the control board first when the flow sensor or a corroded wire connector is the actual problem. A technician should test the sensor output and inspect all wiring connections before replacing any board.
Common Causes
- Failed flow sensor (~50%) The flow sensor itself has internal damage or has worn out and no longer sends a valid signal to the control board.
- Loose or corroded wiring (~30%) The wiring harness between the flow sensor and the control board has loose pins, corrosion, or broken wires that interrupt the signal.
- Extremely low flow rate (~10%) A partially closed valve, clogged strainer, or faucet not opened wide enough can cause the sensor to read erratically if the sensor is already marginal.
- Faulty control board (~10%) The main control board may fail to process the sensor signal even when the sensor and wiring are intact.
Quick Diagnosis
Answer these to narrow it down fast.
Is the main water supply valve fully open and the inlet strainer clean?
No: Open the valve fully and clean the strainer. If the error persists, the sensor or wiring has failed.
When you open a hot faucet wide, does water flow freely at normal pressure?
No: Check for a closed isolation valve, clogged strainer, or a blockage in the cold water inlet.
Is the unit still under warranty?
No: Hire a licensed technician to test the flow sensor, inspect wiring, and replace the sensor if needed.
Step-by-Step Fix
- Turn off power to the tankless heater at the breaker and close the main water supply valve.
- Verify water flow conditions by opening the main supply valve fully, removing and cleaning the inlet strainer, and opening a hot water faucet wide to confirm at least 0.5 gallons per minute.
- Inspect the flow sensor wiring by removing the front cover and checking the harness connector at the flow sensor and control board for loose pins, corrosion, or visible damage.
- Test the flow sensor with a multimeter to check for proper resistance or signal output (consult your model’s service manual for specific values, as these are not standardized).
- Replace the flow sensor if it fails the test or shows no output, following the manufacturer’s installation instructions.
- Check the control board if a new sensor does not clear the error, as the board may be failing to read the sensor signal.
- Contact Navien technical support at 1-800-519-8794 for warranty claims or if the error persists after sensor replacement.
Parts Often Needed
| Part | Notes |
|---|---|
| Water flow sensor | Amazon | Order by your exact Navien model number; some models integrate the sensor into the circulation pump assembly. |
| Wiring harness | Amazon | Required only if the sensor wires are damaged or corroded beyond repair. |
| Main control board | Amazon | Replace only if the sensor and wiring test good but the error persists. |
When to Call a Pro
Navien classifies E039 as a part failure that requires a licensed HVAC or gas fitter. Diagnosing this code involves testing sensor output with a multimeter, tracing control board signals, and working inside a unit that operates on both gas and electricity. The flow sensor is often integrated with other components and requires precise replacement to avoid gas or water leaks. If your unit is under warranty, contact Navien at 1-800-519-8794 before any repair work, as the manufacturer may replace the heater or sensor at no charge. For out-of-warranty units, a professional service call will typically cost $200 to $400 and includes sensor testing, wiring inspection, and replacement of the defective part.
Rough cost: A pro service call runs about $200-400.