What does Error code A3 mean and how to correct it ?

Most likely dust in the combination fan or fan motor is dead

Copy following link for general troubleshoot and error codes:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

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

Error code 36 does not show in Rheem tankless manuals.

Copy following link for general troubleshoot and error codes, and Rheem contact information:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

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

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

Before calling service technician, open following links for Bosch error codes, and contact information, and possible product manual for your model, etc:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Bosch_EA_error_code_troubleshooting.pdf
http://waterheatertimer.org/pdf/Bosch-GWH2400ES-error-codes.pdf
http://waterheatertimer.org/pdf/Bosch_A4-E4_error_code_troubleshooting.pdf

Error code AE is not shown, but error EA means "burner flame not recognized by control unit when water is flowing"

There is also error A3 and A3 E3: jumper connection is not correct

Before calling service technician, open following links for Bosch error codes, and contact information, and possible product manual for your model, etc:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Bosch_EA_error_code_troubleshooting.pdf
http://waterheatertimer.org/pdf/Bosch-GWH2400ES-error-codes.pdf
http://waterheatertimer.org/pdf/Bosch_A4-E4_error_code_troubleshooting.pdf

Error code AE is not shown, but error EA means "burner flame not recognized by control unit when water is flowing"

There is also error A3 and A3 E3: jumper connection is not correct

Here you go, page 30, sounds like it's overheating, possibly a problem with the boiler stat or lack of circulation in the system. http://www.ferroli.co.uk/Product_Documentation/Domestic/Optimax_HE_31_C_Manual.pdf

boilers need a certain amount of water pressure to operate as specified--there may be a pressure regulator valve, allowing a certain amount of water to your boiler, and when water is drawn elsewhere, there may not be enough getting to the boiler---this is one possible solution, but there are easily 20 more, and that is where a trusted plumber is worth it's gold...very difficult to diagnose most plumbing problems without being onsite

you should contact the installer....4 months old should still be under their installation warranty. I looked at the site and it does not mention an A3 code...

Question is though, why would this fail after only six months?