Why am i getting a A3 error code on my bosch aqua star 2700es tankless water heater (LP)? it has worked fine from the time it was installed to now (45 days)

Water in the gas valve is DANGEROUS! The major question is did the water come from a leaking heat exchanger or the product of combustion? If it is from the heat exchanger it may be covered under the warranty (if the installation is proper). If it is from condensate in the flue pipe, it is from an improper installation. If you go to bosch-climate.us you can find a service provider in your area. I service a very large part of NY, some of NJ (Northern) and all of CT.

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

You had service techs and Bosch techs and neither can find a solution.
I assume you want impartial opinion at fixya.
Multiple error codes and a lack of progress by professionals implies PC board failure. BUT there could be multiple problems. Error codes are considered 'start' point for troubleshooting. Beyond that, Bosch should give you contact information to a trained professional service tech >> not just a plumber, but a guy who can do the electronic diagnostic.
Open following link for Bosch troubleshooting resources.
Tankless are high tech gadgets that cannot be exposed to more than 11 grains hardness in the water, and require continual maintenance. The cost of repairs and maintenance will never pay back from the expected $1-8 monthly energy savings.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
You can solve most tankless problems same day for $350.
Then sell the tankless unit back to Lowes or sell it for parts on auction site.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Before calling service technician, open following links for Bosch troubleshooting resources and information that might help narrow down the problem(s):
No error code indicates that first suspect is a plumbing problem.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf

Problem is not clear

Before calling service technician, open following link for Bosch troubleshooting resources including contact information for Bosch:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

I don't have good news.
You are attempting a conversion that local technicians are refusing to do.
There is a reason why we service guys steer clear of some things: liability and continual problems afterward.

Contact Bosch and find the service bulletin for that procedure.
After all this is a highly technical and finicky machine hooked to a gas line located in proximity to people.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

If this procedure is going to cost more than $400-600, and remember there is a risk it will not work afterward or will be a continual problem, then consider installing tank-type LP water heater from local box store. I recommend this.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

For others in same situation: NG to LP conversions are not supported or recommended by every manufacturer. This applies to tank and tankless.
Also check there is a qualified service technician in your area before installing tankless.
Read the manual before buying.
Tankless gas savings are $1-8 per month, but all costs are energy.
Tankless cannot be exposed to more than 11 grains hardness in the water.
Tankless are maintenance heavy, and with expected repairs, maintenance and cost of parts, cost of machine and replacement cost, a simple tank-type water heater is cheaper by far.

Upvote the help.
And 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.

Call qualified service technician, and ask that person questions about maintenance and tips.
Then keep service technician number in rolodex-wallet.

Resourceful do-it-yourselfer will find following links handy.
Tankless are maintenance heavy, and Bosch and Rheem etc say to call service technician for recurring problems. Especially when short-hand troubleshoot section doesn't solve problem.
People spend years learning how to service tankless.

Bosch has online tech support form:
http://forms.boschhotwater.com/Home/BoschTechSupportForm/tabid/614/Default.aspx

BOSCH error codes online:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Bosch manuals:
http://www.boschhotwater.com/HelpfulResources/BrochuresManuals/tabid/354/Default.aspx

Bosch problems at Fixya:
Open following links and read about tankless problems to understand the field and glean useful information.
http://www.fixya.com/support/p949576-bosch_1600h_lp_liquid_propane_aquastar_a/solutions

Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Post question on pro water heater forum: This is where I go to catch up on latest info in always-evolving water heater industry.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

Pay for an answer at fixya:
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.

Call qualified service technician, and ask that person questions about maintenance and tips.
Then keep service technician number in rolodex-wallet.

Resourceful do-it-yourselfer will find following links handy.
Tankless are maintenance heavy, and Bosch and Rheem etc say to call service technician for recurring problems. Especially when short-hand troubleshoot section doesn't solve problem.
People spend years learning how to service tankless.

Bosch has online tech support form:
http://forms.boschhotwater.com/Home/BoschTechSupportForm/tabid/614/Default.aspx

BOSCH error codes online:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Bosch manuals:
http://www.boschhotwater.com/HelpfulResources/BrochuresManuals/tabid/354/Default.aspx

Bosch problems at Fixya:
Open following links and read about tankless problems to understand the field and glean useful information.
http://www.fixya.com/support/p949576-bosch_1600h_lp_liquid_propane_aquastar_a/solutions

Install DIY tank-type gas water heater and avoid the whole headache
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Post question on pro water heater forum: This is where I go to catch up on latest info in always-evolving water heater industry.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

Pay for an answer at fixya:
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.

you will always have this problem in city type of appltcations.there is too much of a flow differation.

unplug power or turn off power,reboot, and try again