Quick answer
Navien error code E004 means false flame detection: the control board sees a flame signal when the burner should be off. The heater will not fire with this code. Make sure it is plugged directly into a properly grounded receptacle, with no extension cord or adapter, and power-cycle once. If E004 returns, have a licensed technician test the flame rod, ground and gas valve.
On a Navien tankless water heater, error code E004 means false flame detection, the NPE-2 manual's term for a flame signal that shows up when no flame should exist. Before each firing cycle the control board checks that the flame circuit is quiet. If it reads a signal with the gas valve closed, it refuses to light and posts E004. Navien revises its code tables between model generations, so confirm against the label inside your unit's cover; that label outranks any website, this one included.
Why a Navien sees a flame that is not there
The flame rod is a metal probe that sits in the burner flame. The board puts a small voltage on it, and a real flame carries a trickle of current from the rod to the grounded burner; that current is the proof of flame. Anything else that lets current leak from the rod to ground looks, to the board, like fire.
That is why E004 is mostly an electrical and moisture fault and only rarely a gas fault. A film of condensate bridging the rod to the burner, a ground that floats, or a board input that has drifted can each fake the signal. The rare exception is the serious one: a gas valve that does not seal, leaving a small real flame after shutdown.
Can I keep using the heater with E004 on the display?
No, and the heater will not let you: it does not fire while the code is present. Cycling the power over and over to coax one more shower out of it is the wrong move. A flame signal with the valve closed has to be explained, not cleared, because one of the possible explanations is gas passing a valve that should be shut.
If there is a gas odour around the unit, skip the troubleshooting entirely. Get everyone out and phone Enbridge Gas once you are outdoors.
The grounded-outlet check Navien asks for
The manual's listed action for E004 is to make sure the unit is plugged into a properly grounded receptacle. In practice that means four things.
- The cord goes straight into a three-prong wall receptacle. No extension cord, no power bar, no two-prong adapter.
- Test the receptacle itself. In pre-war brick houses and 1950s–60s bungalows, three-prong receptacles were often fitted onto old two-wire circuits with no ground behind them, and an inexpensive plug-in tester shows it.
- If the tester reports an open ground, stop there. Correcting the circuit is a job for a licensed electrician under ESA rules, not for the gas technician and not for you.
- With a good ground confirmed, switch the heater off, give it a minute, and switch it on a single time. If the code comes back, do not try again.
Causes of E004 in order of likelihood
- Poor grounding at the receptacle, the cord, or the ground wire inside the cabinet. The flame circuit uses ground as its return path, so a weak ground distorts everything it measures.
- Moisture or condensate on the flame rod. A condensing heater makes water as it runs; a partly blocked condensate drain or a damp, unheated utility space can leave the rod and its ceramic insulator wet enough to conduct.
- A failing control board. The input that reads the flame signal can fail and report a flame that is never there.
- A gas valve that is not sealing. It is rare, and it is the one cause that turns E004 into a safety matter.
What the technician tests when E004 keeps coming back
The visit starts with the cheap possibilities. The technician verifies the ground from the receptacle to the cabinet, pulls the flame rod to check for moisture, cracked ceramic or deposits, and looks for water tracks that point to a condensate problem. The flame signal is then measured with the burner off and with it on.
If the signal is real, the gas valve seat is tested for leakage, which is licensed gas work under a TSSA G2 certificate. A board is replaced only after the rod, the ground and the valve are ruled out, because a new board on a bad ground shows the same code a week later.
An E004 diagnostic visit runs $90–$150 and is credited to the repair if you go ahead. Most tankless repairs land between $150 and $800, and a board or gas valve is quoted in writing before any part is ordered.
Related faults have their own pages: Navien E003 for a burner that never lights and Navien E515 for a control board fault, with the whole table in the Navien error code guide. ZK Mechanical's TSSA-licensed technicians handle tankless water heater repair across Burlington, Oakville, Hamilton and Milton. Call (647) 801-1252 or request a visit with the model number from the rating label.