I have a Paloma PTG-74PVN-2 and we have to turn the hot water on for 2 seconds snap it of and back on for 2 seconds repeat this 5 to 6 times in order to have the hot stay on. If we turn it on once the flame kicks on for a few seconds the off for a few, back on for a few then off with a gas pressure alarm. The pressure is the same as it was when the unit was installed 2.5 years ago.

Even though you are not getting an error code, there are may be some diagnostics in the controller that can identify whether it's an electronic (controller/sensor) or a heat exchanger issue. I would contact Rheem tier-2 support.

Open following link for Rheem/Paloma/Richmond/Ruud tankless troubleshoot resources including error codes, self help, manuals, parts links, tips, and contact information.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Try turning the thermostat uo a small amount and see if this makes a difference. I suspect the control board is bad but I would try adjusting t-stat before I spent money on board or service call. Thank you

that is the water inlet temperature sensor fault.. check and clean scale from sensor or have it replace by a qualified technician

SINCE U DIDN'T PROVIDE A MODEL NUMBER, THE BEST I CAN DO IS GIVE U THEIR PHONE NUMBER:
1 800 873 3507

C8 is the last stop in the point of burner ignition and a total of eight error codes. Error code 12 is a supplemental code that means your LP gas supply is too low...have your LP tank service and try again.

HTH,

Mike...

hi pete here ill try and help this could be 2 things check the sensor on the unit the outher one is a qustion has the gas company changed your meter latley and turned off you gas and not come back to light your poilet because this code also means not enough gas or wrong type of gas

Remove power, disconnect remote terminal. Restore power, open a hot water valve and see if you get any hot water.

HTH,
Mike

I suggest you call tech support. I had a similar problem, and after walking me through a few tests, they determined it was likely a motherboard problem and are sending a new one. At 2 1/2 years, your unit should still be under warranty...

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