Rinnai help, code 12

[MEP001]

Someone asked me to look at his Rinnai heater (I think it was the RL75i, I didn't think to get the model number), if I turn the power off and on it runs and heats as long as there's water flowing, but as soon as flow stops it gives a code 12. The manual has a long list of things to check but everything seems fine, flue is clear, etc. Any ideas what to look for? I know boilers but I've never messed with one of these.

 

[washnvac]

How old is it? Usually it is the flame sensor and electrode after a few years. You can take them both out and sand them. This will buy you a few days until you get the parts. Both pieces together are $10-15 at the plumbing supply house. I have an RL94in, but I think these parts are the same for the 75 unit.

Part # 104000011- flame rod assembly
If the water filter is a couple of years old, I would get that too as they sort of disintegrate over time.
Pat# H98-510-S water filter. (about $4)