We have a Rinnai 556WTA in the basement and it works flawlessly for weeks on end without touching until we head out of town. When we come back it is ALWAYS off with an error code 53. Turn it off/on and it gives us an error code 12. Turn it off/on and it works fine until we go away again.
12 is a gas failure - "Check that gas is on"
53 is a sparker failure - "Service call".
Here is only thing I can think of. When we go away we turn the Rinnai temp down from 64 or 62 to 60. We also turn the heat upstairs way down to like 53 or 55 (from 62). It might cycle on and off a lot more at 60F and maybe the sparker works intermittently? Still does not really explain why we never see the error when we are home...