Water heater work fine on gas but not on elec

Use a digital volt/ohm meter to check voltage at the water heater.

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#pressure

3 flashes, failed pressure switch?
Test pressure switch
Gene

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Call Rheem, they do not support gas change on all models:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Gene

Water Heater Timers Save Money

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following links for lighting and troubleshooting gas water heater.
http://waterheatertimer.org/How-to-light-gas-water-heater.html
Clean air intake, inspect for tripped FV system
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
Post more detailed question on pro water heater forum
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Or call plumber

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following links:
http://waterheatertimer.org/How-to-replace-thermocouple.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Open following link for troubleshoot error codes showing 7 flashes on Honeywell gas control valve is system lockout due to flammable vapor:
http://waterheatertimer.org/pdf/Honeywell-gas-control_error-Damper-series.pdf
The FV system in Rheem can be tripped by presence of flammable vapors such as gasoline or paint fumes etc, or it can be tripped if water heater was dropped, or mis-manufactured, or if gas control valve is defective, or if water heater is not getting adequate fresh air, or there is inadequate venting of combustion by-product.
When combustion chamber overheats, then glass vial breaks and damper shuts off air supply to water heater and it will not light.
Open following link for possible help fixing the problem:
http://waterheatertimer.org/How-to-repair-Rheem-FVIR.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

More information is needed.
You posted under electric water heater.
Are you trying to light a gas water heater?
Each water heater is different depending on gas control valve.
Instructions for each gas water heater are printed on front of water heater at gas valve.

Add a comment and describe the gas valve, describe what the instructions say, and describe what general problem you are having for possible asistance.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Once the hot and cold pipes are connected to water heater, then open bathtub spout on hot side, turn water on so tank begins to fill.
After tub spout is running full stream without air, then water heater is full and it is safe to turn on the heater.

Turning on gas water heater before tank is full can damage tank.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Turning on electric heater before tank is full will burn out elements. Replace elements.
http://waterheatertimer.org/How-it-works.html

Add a comment for more free help.
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.

if its gas, it has a pilot light. the most likely thing is the thermal coupling is bad. if it is bad, burner flame will not turn on. it senses the tempurature to send gas to light burner.

THE LIGHTED SWITCH SHOULD BE THE 12V ENERGY SOURCE TO ACTIVATE THE LP SIDE OF THE HEATER. THERE SHOULDN'T BE A LIGHTED SWITCH FOR THE 110 SIDE. THERE SHOULD BE A SWITCH OR A COMBINATION OF SWITCHES. ONE ON THE WALL LIKE A LIGHT SWITCH AND ONE MOUNTED BEHIND THE HEATER.