I have a Rheem Tankless Indoor RTG-74-PVN-2 that has been shutting off every now and then we would pull the power cord and wait for a few seconds and connect it back and wait for a seconds and push the power on and it would turn on. But this mourning it would turn on after we pulled the cord and pushed the on button it would reset fine then we would turn on the water and it start flashing AO then 21. I need hot water

http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Fuse?
Wrong polarity on outlet
Unit not grounded
bad PC board

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

3" category III stainless for special venting

Tanklessking shows Category III stainless steel vent kit for this model
3" vent
http://tanklessonline.com/rheem-water-heaters-RTG42.html

ptg-42-pvn has minimum 31,000BTU and max 118,000BTU
Max water temperature 120 degrees F
Spec sheet says: 3" special venting
http://www.palomatankless.com/products/42/specifications.html

Hart and cooley show special vent 3" stainless thru-the-wall vent
http://www.hartandcooley.com/specialgasvent/PDF/851515%20Tankless%20Water%20Heater%20Installation%20Vent%20Kits%20sell%20sheet%20-%20HC%20-%20low%20res.pdf

Hot water sources shows thru-wall or thru-roof
http://www.hotwatersource.com/RTG42PVN-Rheem-Indoor-Natural-Gas-Tankless-Water-Heater/RTG42PVN,default,pd.html

http://waterheatertimer.org/pdf/Tankless-roof-vent-installation.pdf

Open following links for Rheem troubleshooting resources, including Rheem chat, error codes, and possible manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Error codes are considered start-point for troubleshoot.
Error code 24 is malfunction of operational switch: press max or min button on PC board to reset, -or- replace PC board.
Max-min button is sometimes located above dip switches, and can be 1 button that is rotated with screwdriver or 2 separate buttons. Look at wiring diagram in manual that came with product.

Fix most tankless problems yourself, same day, for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Does it have a battery in it? check that

Before calling service technician, open following link for Rheem troubleshoot resources including error codes and typical manuals.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Recall website:
http://www.tankless-recall.com/tanklessSearch.aspx
Error code C2 13 call Rheem.

Read manual cover-to-cover to make sure your tankless water heater receives yearly maintenance required for unit to continue working.
Tankless are maintenance heavy which can quickly eliminate any promised energy savings unless you de-lime and troubleshoot and read tips-and-tricks others have written and glean information from service technician who visits your house. Mostly glean how much tankless repairs and parts cost vrs buying whole new tank-type gas water heater at local box store, since all costs are energy.

This is something you will have to contact rheem about due to the fact you don't have a fault code

Some error codes cannot be reset...Example would be P/S failure on the PCB. Any other error, if you remove power for approx. 2 min - error should clear. - if not you need to contact Paloma/Rheem customer (800-432-8373) support

Contact Rheem; the Burner control unit has lost comm with the remote...the part is warranteed for up to five years.

Check to make sure wire is solid copper thermostat wire and not stranded

Error code 13 is "oxygen depletion sensor" activated.
The first number is a sequence indicator.
If you need the training manual I have it and can send it to you.
I got Rheem to send it to me a while back and it is a big help when working with these units.
It covers th RTG-42, 53, 66 and 74.