Getting an A7 error code and no hot water - Bosch AquaStar 2400ES-NG

Hot water sensor defective

I just had this exact problem. It kept getting worse until we had no water. Our Rinnai tankless hot water heater started the same way though, with hot water for a few minutes and then would shut off with an error code 10. We just had to shut the hot water off and then back on, wait a minute or so and we had hot water back for a few minutes. Our unit would come on with a loud vibrating noise and remain until it shut off with an error code 10 (air flow fault). The solution was to remove the blower fan and blow out the squirrel cage. Ours had moths in it. We also vacuumed out the fan housing. Be careful to mark any wires you disconnect and take pictures before removal. Our unit is now almost silent when it comes on and supplies endless hot water.

Here is the troubleshooting guide for the A7 code:
http://www.bosch-climate.us/files/201109161634300.TWH-G2-31_A7_error_code_troubleshooting.pdf

Open following links for water heater problems with no error code:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Low-hot-water-pressure.html
For tankless:
http://waterheatertimer.org/pdf/No-Error-Code.pdf

Questions might be asking for a manual, and asks what error A7 means, and asks how to fix error problem, but model number of Bosch heater is not given.
Error codes are start-point for troubleshooting, and can include multiple causes.
So following answer provides general do-it-yourself resources, without saying that problem can be 100% diagnosed or fixed by homeowner.

1) Error code A7 is outlet temperature sensor fault.
Problems include: need for descaling, dirty inlet water filter, debris in faucet aerators, dirty wiring terminals, bad wiring harness, or bad temperature sensor.
http://waterheatertimer.org/pdf/Bosch-GWH2400ES-error-codes.pdf

2) Before calling service technician who can test failed parts and catch tankless up on yearly maintenance, open following links for Bosch resources including error codes, phone number, possible manual, and descaling instructions, and cleaning inlet filter:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Rheem-De-lime-tankless.pdf
http://www.boschhotwater.com/Portals/7/TechBulletins/TWH-G1-12_Cleaning_inlet_filter.pdf

3) Product manual: Question posted under model 125HX-LP
Open following link for possible manual:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/125HXTechSupport/InstallationManuals/tabid/542/Default.aspx

4) Do it yourself service bullitens for 125HX-LP
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/125HXTechSupport/ServiceBulletins/tabid/548/Default.aspx

5) Take a moment and rate answer:
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.

Before calling service technician, open following links for Bosch troubleshoot resources:
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Error codes are start-point for troubleshoot.
A7 is temperature sensor fault.
This can be bad wiring, corroded wiring, failed sensor, very high temperatures due to other part failure.
Other cause can be water filter clogged, lime and scale inside heat exchanger, faucet aerators clogged, or combination of problems listed.

Each tankless manual across all manufacturers suggest maintenance schedule: clean water filter monthly, delime heat exchanger yearly, and yearly service of burner and other parts by trained service technician which can address cause of most error codes, and might solve some problems before they happen.

If tankless becomes chronic issue, solve problem yourself, same day, for about $350.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Before calling service technician, open following link for troubleshooting resources including error codes, manual, links to bosch, phone number, :
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

A7 is outlet temperature sensor fault.
Temperature below freezing or above 210 degrees F

1) Check red wire connections at hot water temperature sensor. Clean terminals with an eraser. If badly corroded, replace sensor and wire harness. (See Fig. 63, page 46 in manual).
2) Sensor may trip if water temperature drops below freezing to protect heater from freezing conditions. Any damage due to freezing conditions is not covered under warranty.
3) Clean inlet filter screen and clear any debris out of shower heads, faucet aerators and whole house filters.
4) In areas where water has a high mineral content, periodic descaling may be necessary.

Use following link for Rheem tankless troubleshoot: http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

I believe error code C7 is same as error code 76

Add a comment for more free help.
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.

Here is the A7 code for this heater:

Outlet temperature sensor fault

(Temperature below freezing or

above 210°F).

1. Check red wire connections at hot water temperature sensor.

Clean terminals with an eraser. If badly corroded, replace sensor

and wire harness. See Fig. 65, page 48.

2. Sensor may trip if water temperature drops below freezing to

protect heater from freezing conditions. Any damage due to

freezing conditions is not covered under warranty.

3. Clean inlet filter screen and clear any debris out of shower

heads, faucet aerators and whole house filters.

4. In areas where water has a high mineral content, periodic
descaling may be necessary.

I would be very surprised if your water temp is below freezing, even in Alaska, check it with a thermometer. Well water is usually around 45 degrees F.
Let me know,
Gary