Bosch 125b-ng water heather has low flame only

Question edited for clarity and add maker.

If this is in normal operation, turn off the gas and water and call a gas registered engineer. If you are turning off the water because you are going on vacation, turn off both the water and the gas. It may be sensing water flow because the pressure has dropped.

Bosch 1600H NG??? I suppose. If so you need a digital manometer.
Take a static (off) reading then leave it hooked up and start flow. Watch pressure draw down and you should see the heater draw Water Column down below 5" as the pocket of NG in the pipe just before the unit draws down to minimal to keep heater lite. In that case problem in gas line size/gas line run length, Meter pressure.
If you have and maintain in operation 5" WC then is probably the water valve needs rebuilt along with a new diaphgram and also the water filter should be replaced too. This occurs esp after 2-3 year as Bosch specs rebuild at 3 yrs on the Maint Schedule. Also clean all the faucet aerators for good flow above 1.25 GPM Min at all faucets.
Under a single faucet flow (laundry sink) of 2.25 GPM the flame bed of the Unit should be 1"+ high and uniformly Blue.
I assume at this time you either have a Maint problem or you could be overflowing the heater's Winter capacity. This unit @ 70F rise only makes about 2.2 GPM. It's strictly a single use faucet heater at 117,000 BTU.
When you do the Maint look to see if there is corrosion between the GV pushrod and the water valve pushrod. Notice there is a huge mis- match in the 2 rods sizes.These units occasionally (more the 125B) corrode there and the GV will seize on and boil the heater dry and open a seam so you really want to maintain it to Bosch Specs and Schedule.
Note: This is a gas appliance & requires skill and knowledge to service. While Bosch may say you can DIY, I'd readily advise a Bosch approved Service Tech or Plumber do any rebuild. The Digital Manometer alone is a $100-150.00 item and it's the ONLY way to get accurate gas pressure readings.

1) If tankless is not showing error code, the problem might be single-handled faucet cartridge:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

2) Catch up on maintenance, such as cleaning combustion parts, have gas pressure checked, etc.

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

Bosch 1-866-642-3198; 800-642-3111
Copy following links to narrow down cause for EA error code:
http://waterheatertimer.org/pdf/Bosch_EA_error_code_troubleshooting.pdf
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Aqua star service bulletins
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/

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

Thermocouple has 2 ends. One end attaches to gas control valve, other end sits inside the pilot flame.
Flame heats thermocouple, causing tiny electrical current that is detected inside the gas control valve and keeps the gas valve open so water heater can turn on when thermostat calls for heating.
If pilot goes out, then no electric current reaches gas control, and all gas to burner and pilot is shut off.

If pex line burst, that implies very high pressure, caused by closed system with check valve that stops expansion, or possible clog in the water line, or high local water pressure, or clogged factory-installed heat traps.
Put pressure gauge like BTG100 on the drain valve to check pressure, should not exceed 80psi, install pressure reducing valve on cold water line, install expansion tank on cold water line. Remove heat traps and install ordinary pex-lined di-electric nipples from hardware store.

If the water was super hot, then overheating could have caused the pipe explosion.
However the gas control thermostat is ECO protected, and if ECO (energy cut off) trips because of high heat detected inside the tank or combustion chamber, the gas control will trip off and no longer work, gas control needs to be replaced. Some newer electronic gas controls can be reset, but that type heater uses a thermopile instead of thermocouple.
Thermopile is basically a group of thermocouples, that product more electric current that is used to power electronics inside the gas control valve, which issues an error code on the gas control, and some day will send a text message each time out-of-range event occurs.

Copy following links for troubleshoot resources, including troubleshoot by type of gas control valve:
http://weaterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/pdf/How-to-replace-water-heater-gas-valve.pdf

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

Sounds like hard water. Service the water valve. Parts are available from your local Bosch Service Provider (Contact Bosch if you don't know who the service provider is).

Take the thermocouple out and match it up at any hard wear store. Maybe $10. Replace it .Make sure the flame is clean and hitting the rod in the center center. Ajust (Bend) accordingly.

I found the manufactures website and read this specific tech bulletin. I will be replacing or repairing my water value soon.

http://www.boschhotwater.com/Portals/7/TechBulletins/TWH-G1-25_Water_valve_rebuild_B.pdf

The bulletin said the following:
If the burners fail to go off immediately when the
water is turned off, usually due to mineralization in
the water valve on older heaters. To test for this
possiblilty, run hot water at a faucet, then shut off
the cold inlet supply and observe if the burners go
off immediately. If they do not the Aquastar will
overheat and shut down on safety depending on
the delay.

it sounds to me like the dip tube has failed. this is a plastic tube that is inside the water heater on the cold side. It is designed to put the incoming cold water at the bottem of the tank allowing heated water out at the top. when this tube deteriorates the cold water mixes at the top of the tank instead of the bottom. a new dip tube is about 5 bucks but must be inserted on the cold side by someone with plumbing experience.

Replace the thermocouple.

Thats the flame sensor.

If the gas valve is not sensing flame it will not stay lit.

T-couple's are about 9 bucks.