Water heater does not come on all of the time.it will give you an error code ao-21 or c2-11

page 16 in instructions that came with unit.

Copy and paste https://www.aosmithatlowes.com/media/1378/tankless-electric-2-4-chamber-installation-manual-and-use-and-care-guide-100285618.pdf

Here you are Dorothy

AO Smith code 991 Solution for error code 991
Check the gas type of the water heater. Inspect the environment around the water heater. ... Check if there is grease and/or dirt in the burner (Part #101) and the fan motor (Part #103), especially if the water heater has been installed in a contaminated area.

Error code '991' for type ATI-110C-N by AO Smith. Cause and ...

More help here, but you probably need a service call, especially if it is a gas water heater.

https://www.google.com/search?q=error+code+of+991

..

Error codes for both types of honeywell gas control valves appear on following link...
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#honeywell

Whirlpool no longer sells water heaters ... but in the past, they sold the AO Smith brand flue damper and atmospheric heaters ... both use different Honeywell gas controls.

Maybe this error code chart for AO Smith Power vent will help you figure it out. Also link to AO Smith website. Good luck. J

https://www.hotwater.com/support/gpvh-50

Copy following link
http://waterheatertimer.org/pdf/AO-Smith-heat-pump-service-manual-321547.pdf

Contact AO Smith for qualified technician in area.
Don't buy expensive water heaters.

Gene
h

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

Call manufacturer.
State water heaters are made by AO Smith, and AO website posts service manuals with each product on literature tab.

Gene
h

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

Most recent power shot has intellivent gas control valve:
Copy following link to troubleshoot by type of gas control valve:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
Generally you can wake up the control by holding down both temperature adjustment buttons for 1 second then release.
Otherwise, add a comment and give more information, if this is new heater or old heater that stopped working, and what error code if any you are experiencing.
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Post question with more detail on pro water heater formula.
More information is needed... and fixya truncates free questions so not sure I am seeing whole problem

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 troubleshooting resources including error codes, Rheem chat and phone number, possible instruction manual, maintenance manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start point for troubleshooting.
Error code 21 is "malfunction of air intake filter switch"
Make sure air filter door is properly seated.
Make sure front panel is properly installed.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

This means the air intake cover problem.
21
Malfunction of Air Intake Filter Switch Ensure air filter door is properly seated. Ensure front panel is properly installed.
Error codes here: http://www.palomastore.com/paloma_error_codes.html

its most likely diplaying a code... is it code 21? that usually will follow ao. first make sure the door is securely on, if not make sure it contacts the switch