My water heater is flashing a cod 2 then 3 how can i reset this i lost ths manual

Rheem RTG2 42 Troubleshooting Manual Error Code 99

Error code means that you have electronic gas control valve.

Copy following link to identify gas control valve, read error code meanings for each, and download service manual relevant to each gas control, but not specifically Rheem since rheem does not post service manuals.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

The fact that water heater restarts, means that water heater is getting gas, getting sufficient air for combustion, thermister is ok, gas pressure is good, etc.
The fact that you unplug and replug water heater and it works, shows that you have power vent model.
Since water heater worked correctly before, 2 flashes with Rheem power vent model could mean that pressure switch has failed.

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

Type of gas control valve is not known.
Brand of water heater is not known.
FV event can mean that air intake is clogged, gas control valve has gone bad, or FV system has tripped, or TCO has tripped ... or all of above.
Clean air intake, download manual from manufacturer and read troubleshoot section, call manufacturer.
Copy following links for troubleshoot resources
http://waterheatertimer.org/How-to-repair-Rheem-FVIR.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://www.thetankatwaterheaterrescue.com/forums/forum3/

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

Before calling service technician, open following link for Rheem tankless troubleshoot resources, including on-line chat
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Rheem technical service bulletin for de-liming:
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf

De-liming Manual says:
10. Turn on the water shutoff valves, run a hot water tap to
purge any air from the water lines and check the system
for any leaks.
11. Turn on gas, electricity, and remote control thermostat
and set the desired temperature.
12. Run hot water from several locations to check the operation
and performance of the water heater.
If you have difficulties please contact the Technical Service
Department at 1-800-432-8373 or your local service agent.

2 green flashes is 'High Water Temp.'
http://waterheatertimer.org/Whirlpool-Energy-Smart-electric.html

Typical energy smart manual:
http://waterheatertimer.org/pdf/Whirlpool-energy-smart-electric-manual.pdf

3 flashes is Temperature Sensor failure.
4 flashes is Upper element
5 flashes is Lower element

Troubleshoot section says:
1) Make sure access doors and insulation are installed.
2) Check elements for resistance of 5-25 ohms (if element has a short in center of element, it might not trip breaker)
http://waterheatertimer.org/How-to-test-water-heater-element.html
3) replace element and reset control system.
Buy ordinary 4500 Watt element from hardware store.
http://waterheatertimer.org/How-to-replace-water-heater-element.html

4) I also suspect the self-diagnostic control box.
Manual says: Repair parts may be ordered through your plumber,
local distributor, home improvement center, or by calling
1-877-817-6750.
You need serial number, product number and model number from label on side of tank.

Is it flashing 2-3? Try this: You can reset the control by unplugging the blower during start up, leave unplugged for at least 5 seconds, plug back in and within 30 seconds cycle temp control from hot to low 6 times.

2 flashes = high water temperature (the water heater comes set at 120, and you can set it lower or higher)
3 flashes = sensor failure (temperature sensor similar to a thermostat on regular water heater)
4 flashes = upper element
5 flashes = lower element
Links below are for Whirlpool manuals
http://www.whirlpoolwaterheaters.com/support/Manuals.aspx
http://waterheatertimer.org/pdf/Whirlpool-energy-smart-electric-manual.pdf

The water heater is working right now.
If the elements go out, they are available at any hardware store.
The temperature sensor is easily replaced and can be ordered from Whirlpool.
You might want an extra temperature sensor on-hand so repairs are not delayed.
Link below shows the product in detail.
http://waterheatertimer.org/Whirlpool-Energy-Smart-electric.html
Let the water heater do its job until it fails.

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.

BOSH AQ 250 On demand propane water heater ran out of propane, got gas now, but get EC flashing on the display and the unit won't fire? Lost manual, any suggestions. Thanks, Jim

The water heater is locked out. The following process should reset it.
1. Reapply power to the water heater and wait for 3 seconds for the system self-diagnostic check.
2. Within the first 30 seconds, the temperature dial must be rotated back and forth crossing the 120
degree mark at least six (6) times. This will reset the control and display a normal flash code
(slow blinking).