it is possible it needs a reset.
1. turn off the breaker to make sure there is no power to the unit
2. open the top cover and there will be a red button like this
http://www.tonyhoover.com/wp-content/uploads/2010/03/water-heater-upper-thermostat.jpg
push it. It may have a plastic cover over or around it, leave this in place it is insulation for the wiring.
close it up carefully and then turn on the power and see what happens.
Probably a faulty thermostat but a possibility, depending on design and area that heavy internal scale deposits are preventing the thermostat doing a good job.
You posted under typical Rheem tank-type electric water heater, but don't say what service you want to perform. Listed below are typical actions one takes with electric water heater:
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There is no on/off switch. With the circuit breaker off you hook up the power lines to it, and then restore power. The thermostat will take care of turning unit on and off as water temps dictate.
the age of all water heaters is listed under the serial #, not model #. you will have to call a plumbing supply house that deals in your brand of waterheater, a.o. smith, ruud, bradfordwhite,etc.
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You either have water with a high sulfur content or need to flush your heater. you should flush your heater every year anyway. you can turn off the water and electric to the heater. and totally drain the tank when it is totally drained add more water and drain some more. then refill the tank and run a hot water faucet the tub is best to eliminate any air in the system then turn the electric back on this should solve the problem if it does not you probably have sulfur in the water and would need water treatment or filters to help
it does have 2 elements they are located under the 2 panels on the heater you will find them under the covers on the front they will be connected to the thermostats follow the wiring. besure to turn off the power before working on it and use a tester to double check
First, let me say that no Rheem electric water heater that I am aware of has only 1 element.
You may have to search - but there should be one about 8 inches from the top and another behind a panel about 10 inches above the bottom. Please search again.
It is NOT possible to add an element to a "used" water heater. The inner and outer liners must both be sealed and that is done at the factory.
I would first look at the owners manual and or the schematics. You can probably find it on line if you have the model and serial number. I would then call a Rheem dealer or contact the company to see what that error code is. Personally I wouldn't try to fix it myself because it's gas and if you screw up you could blow youself up or burn the house down. That's why I buy electric water heaters and a water softener. I may pay a little more in electric but with the softner to remove the lime, I never have to change an element, only add salt about every 3 months.