I'm out the door right now to tell the homeowners once again that the gas was never calculated correctly, the installer did a take off from the FAU on what was already a 3/4" X 65' line from the meter. they have had a repeated 03E error code and I've explained, the flame rectifier needs to see .5 VDC for 1.5 sec or the gas valve closes and will attempt this cycle 3 times before the error code comes on and you either have to trace the problem in this case to original installation or in the box and honestly I find the problems generally out side the heater. BTW, the original installed terminated the gas with a 3/4" copper water flex.
In my area of the central california coast, gas regulators in the marine influence that have been in service for decades, never had to open to deliver high demand and most I find are stuck partially closed from salt air. When found, these are replaced with new ones from the ultility. This same condition cause a growl in the first minute of ignition cycle and complaints are again aimed at the unit instead of the problem confusing the symptom with the tankless it's self.
If you are going to install a Navien as a professional, purchase and learn to use a dual port manometer to check low and high fire cycles before finishing the job not just take the heater out, install it and leave. If not, you leave service providers like me with more work than I need or want.