Hi yes it is the oxygen sensor but I cannot find where it is . looked on the rheem site . There is no picture of what it looks like.
SOURCE: Rheem RTG 53 PVN with fault codes C4 or C7
First off, if you don't have a water softener supplying the tankless you have voided your warranty; it's in the fine print. The mineral deposits build up and clog the heat exchanger; that may not be the problem because you're not having water pressure issues. Furthermore, you don't need the control unit for the tankless to operate; remove the control unit and unplug / shut off the breaker for atleast 3 minutes. Turn the power back on and see if that resets the unit; this may get your hot water back until you can determine the error code. I will look up the Rinnai error codes tomorrow and get back to you on this since my experience with Rinnai products is limited.
SOURCE: Rheem RTG 53 PVN with fault codes C4 or C7
I had the same problem with my tankless waterheater (flashing c4 76 alternately). I called Rheem
tech support and they had me 1 power down the remotepanel 2 unplug the unit from the wall 3 remove the cover 4 remove the wires for the remote panel from the circuit board and pull the twisted pair that goes to the remote out from the bottom of the unit. 5 reinstall the cover 6 plug the unit back
into the wall and turn on the hot water see if it works. It did in my case and they are sending me a new remote panel. I hope it helps.........
SOURCE: how to correct error code C-7 on my RTG-74PVN Rheem Pronto
I had the same problem and called the manufacturer. The error code C-8 was followed by the number 76. They told me to disconnect the thermostat/digital display from the main unit. This was fairly simple to do by removing the front panel. The unit works steady now without shutting down after a few minutes. The help desk told me that the main circuit board needs to be replaced for the unit to work with the display and that they would mail me one asap. At least we have hot water again.
Oakville, Ontario
SOURCE: Rheem Pronto RTG-74 error code C4
how is the water pressure? it has to have at least 65 lbs min to work if it's low on pressure it might be the filters are clogged on the unit above the shut offs, is there anything blocking the flue pipe out side your house, it has a fresh air return and it has to come in with fresh air or the unit will over heat, there for shut down. have you added any other gas units in the house tha might be taking the gas from the unit not giving it enough gas to operate, with what you wrote these are a few item that I can think of for you to check that would cause a tankless water heater to fail, the vent has to be atleast 60 inches from any other exhaust vent from any other item on the out side
SOURCE: Heater shuts down with C4-13 code
your not getting enough fresh air to it. where is it located? you need 1 square inch per 1000 btu's. it's having irregular combusion.
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