Is there an online manual for installation and service of the Navien gas tankless water heater CR-250?

SOURCE: No hot water from tankless water heater. Bliniking Readout

http://www.rheemtankless.com/content/tankless/rheem/help.shtml

This is there website.Above.
Use the live chat to ask them, the code you listed does not seem to correspond to there code list, the only code that does is code 76,
which is a problem with the remote control not talking to the unit.
So your best bet is to tell them the error.

SOURCE: Paloma PTG 74PVN Tankless Water Heater

did you check the filters on the the water valves, i am not exactly sure about your codes that are shoeing but if you could read it and post them, i know on some of the units 76 means there is low water pressure and the filters are behind 2 caps one on each side above the shut off valve take off the caps and remove the screens, clean them and replace them ,, make sure you turn off the water on both side first or you will get wet

SOURCE: rheem tankless water heater wont lite

the system was not sized properly, you need to have a 3/4 gas line to the unit for the volume of gas it needs, the tankless water heaters need to have every bit of gas that is says, it sounds like who ever installed di not make sure you had enough gas comming in the house, you might have to either have a new meter installed or run the gas lind directly from the meter to the water heater and cap off the gas line thats there

SOURCE: Paloma tankless, gas, water heater, error code C7 76

If you look on the left hand side of the burner control module you'll see 4 capacitors - the capacitors have most likely failed...pretty sure anything code 76 is P/S failure. It may also be covered for manufactures defects. If the cap is blown..the top will be slightly raised as apposed to flat like the other nearby caps - they are called top blow capacitors...the damage should be evident.

SOURCE: heater will not ignite

The sensor that allows the pilot to ignite is at fault
replace the probe that points at the pilot light which is the sensor