Getting an error code

Gas issue, check your gas supply to the heater. If the unit has been sitting a while, you may have debris in the burner. Spiders and mud wasps love the smell of gas and nest inside unused heaters blocking the gas. Check your exhaust vent for obstruction. If you can, open the front of the heater by removing the two top screws on top right and left sides of heater, and unplug the light as the panel opens. You will find two small rectangular pieces of metal held on the heater with one screw each. Take these off and it will have a small window you will be able to see inside the heater. Cycle the heater and look for blue flame between the two windows. If you don't get any but you hear clicking coming from the top right side of the heater, you have an obstruction or the igniter is not sparking. Check all the wires behind the plastic on the front of the heat exchanger and follow them back to the circuit board and check there. If you get flame but goes out in a second or two, the flame rod is dirty or bad and you will need a technician who is familiar with the Rinnai. Best I can do without being there! Good luck>