Navien NPE-2 Code E773
Using the wrong gas. The heater has determined it is being supplied with a gas it is not set up for — natural gas in a propane-configured unit, or the reverse. This is another NPE-2 addition; the original NPE has no code for it. Navien's instruction is to contact technical support.
Call a proThe verdict
Call a licensed plumber
Stop using it. A burner sized for one fuel running on the other is the wrong air-to-fuel ratio by a wide margin, in both directions. The fix is normally a conversion done properly, or a configuration corrected — but that determination belongs to a qualified gas technician, not to a reset.
Typical cost: $150–$900
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NPE-2 — not averaged across the brand.
Source: Navien NPE-2 User's Information Manual (p. 18), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Turn the heater off at the controller and close the manual gas valve below it.
- If you smell gas, leave the building and call your gas utility from outside.
- Find out what your house actually runs on: natural gas from a meter, or propane from a tank.
- If the unit was recently installed, converted, or moved, contact that installer — this code points straight at that work.
Frequently asked questions
How can the heater tell which gas it is getting?
Not by analysing it. It infers the mismatch from how combustion behaves against what it was configured to expect — which is also why the code can appear some time after a conversion rather than immediately.
Is this the same as E788?
Related but not identical. E788 is a mismatch between the gas type set on the panel DIP switches and the panel itself — a configuration disagreement. E773 is the heater concluding the gas being supplied is wrong for it.