My propane 240 unit has been a problem virtually from the beginning.
The system has a safety feature that won't allow it to start unless at least 1/2 gal is flowing through it per minute. Not a problem on its face, but the way the unit tells how much water is passing through is by the use of a flow meter. A flow meter is a small plastic turbine inside a tube; as water passes through the tube, it spins it. As it spins, it swings a magnet(I surmise it is a magnet) round and round. There is a pickup on the outside of the tube that records each time the magnet passes by, the computer then makes a calculation with that figure which tells the unit how much water is passing through.
The "flow sensor" is a terribly weak link in the system. Sometimes it turns, sometimes it doesn't, and you don't know which until you get into the shower. If you will look around the internet you will find that this is a very common problem and there is no fix for it. It is not going to be fixed unless Navien will completely redesign its flow meter. Rennai does not have this problem, though I dont know how they measure flow.
When I first started dealing with Naviens' customer service, they were rude, arrogant, and incompetent. Eventually I did get a Navien Tech supervisor, Bryan Coccia, he at least has done his best to try and resolve the problem, and he is still trying, but at this point, I dont think there is anything he can do about it other than just give me a refund, because no matter how many flow sensors he gives me, or if he replaces the whole unit, it will still have the Navien flow sensor, and the Navien flow sensor, as constructed, will never work properly.
I would like to just get my money back and go buy a Rennai, which does not have a similar problem. Had I looked up this product prior to purchase, I would never have purchased it, but my plumber picked it, not I , and I assumed he knew what he was doing, but I now think that he was just angling for a trip to Hawaii or some such reward from the factory to reward him for pushing their worthless product. This last is just a guess.