Quick answer
Navien error code E407 means the hot water outlet thermistor is open or short-circuited: the heater has lost its outlet temperature reading. It stops heating, and there is no safe way to run around that. Power-cycle the unit once; nothing else is user-serviceable. If E407 returns, call right away with the model and serial number, since Navien directs owners to technical support for this code.
Navien error code E407 means hot water outlet thermistor open or short circuit. The thermistor is the small temperature sensor on the hot outlet, and the control board uses its reading to decide how hard to fire. When the circuit reads open (a break) or shorted, the board has no outlet temperature to work from and shuts heating down. That wording comes from the NPE-2 user manual; codes vary by model, and the label inside the front panel overrides any list you find online.
Can the heater run without the outlet thermistor?
No. A tankless heater that cannot read its outlet temperature cannot limit it, and water leaving at an uncontrolled temperature is a scald risk at every tap. The control refuses to heat for that reason, and nothing on the front panel lets you run around the fault. Do not unplug sensors or swap connectors to test a theory; a wrong reading is worse than no reading.
The only homeowner step for E407
Power-cycle once. Turn the heater off at the front panel, wait a minute, and turn it back on. A momentary glitch can latch the code, and a single restart separates the glitch from the real fault.
Nothing else here is user-serviceable. Navien's manual gives one instruction for E407: contact technical support. If the code is back on the display after the restart, or returns over the next few days, stop there.
What usually fails behind an E407
- The outlet thermistor itself. These sensors sit in hot water for years and eventually fail open or short.
- A loose or corroded connector. Moisture inside the cabinet turns the terminals green, and a connector left half-seated after an earlier service visit can work for months before vibration finishes the job.
- A damaged wiring harness, where a wire has rubbed through on a metal edge or been pinched behind a panel.
- The control board, less often. If the sensor and the wiring test good, the board input that reads them is what remains.
Cabinet moisture is the thread running through most of these. A heater in a damp, unheated space, or one with a slow drip from a fitting above the wiring, corrodes connectors long before the sensor wears out. Tell the technician about any water you have seen inside or under the unit, and whether the code first appeared after a power outage or plumbing work near the heater.
Before you call: model, serial number and who owns the heater
Have the model and serial number ready; both are printed on the rating label on the heater's cabinet. Navien's manual says to contact technical support first for any warranty service, and support will ask for those numbers before anything else. Your installer's invoice with the install date helps too, since that date is where the warranty period starts counting.
Then check whether the heater is yours. Many tankless units in Ontario are rentals, and a rented heater is the rental company's to repair: call the number on its sticker, not a contractor. Who pays for repairs on a rented water heater explains how that works.
What the technician does for a thermistor fault
The diagnosis is a meter job. With the power off, the technician unplugs the outlet thermistor and measures its resistance, comparing it with the value the manufacturer publishes for the water temperature at that moment. An open or shorted reading condemns the sensor; a good reading sends the technician along the harness to the board, checking each connector for corrosion on the way.
- Diagnostic visit for a sensor fault: $90–$150, deducted from the repair if you go ahead with it.
- Sensor, connector and harness repairs sit within the usual $150–$800 range for tankless work.
- A control board, if it comes to that, is priced in writing before it is ordered, and any warranty part is confirmed with Navien first.
If the display shows a board fault instead, see Navien E515; for a heater that overheats while its sensors still read, see Navien E016. All the codes are collected in the Navien error code guide. For an owned heater anywhere from Hamilton to Etobicoke, ZK Mechanical does tankless water heater repair; call (647) 801-1252 or book a visit and include the serial number.