The red button on left side of Rheem electric hot water heater always needs to be reset.

SOURCE: No hot water, Rheem Model RR612T, Electric Hot Water Heater, 60 g

If you don't have current, 0v from terminal to terminal, then you don't have voltage to the elements. Replace the thermostat but check that you have 220v to the thermostat. It is possible that your breakers to the unit are either bad or tripped.

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 tankless water heater wont lite

the system was not sized properly, you need to have a 3/4 gas line to the unit for the volume of gas it needs, the tankless water heaters need to have every bit of gas that is says, it sounds like who ever installed di not make sure you had enough gas comming in the house, you might have to either have a new meter installed or run the gas lind directly from the meter to the water heater and cap off the gas line thats there

SOURCE: 1 unit intermittently shuts off and other won't ignite

if you have the exhaust run together it will not light up,, they have to be seperate,, the computer board sences the unburnt gasses and will not allow it to start,,

SOURCE: Rheem Power Vent 2 water heater is flashing 4

The flashing light is a trouble sequence light. Find your manual to see what 4 flashes means. It's usually attached to the side of the water heater tank. Try a quick fix by unplugging the power vent and see what happens.