Bosch gas heater is firing up and hot water is coming through but then cuts out after 1min water pressure drops and the burner cuts out

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.

possible faulty pressure switch connected to the water line and that is failing to turn on the main gas tap solenoid
best have it checked by an accredited gas fitter or service center for the make

Tankless?... call service technician

Tank? clean arrestor screen, replace gas valve thermostat
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

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

probably the chamber is burned out --- call Bosch with the problem and they will walk you through the sequence of testing. Call them

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

Probably the burner and parts need to be cleaned yearly as recommended in most gas water heater manuals. Both tank and tankless today recommend same.
Open following link for Bosch troubleshoot resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Ok the way this system works is from a pressure and flow switch. The pressure switch picks up the slight drop in pressure when the hot side water is turned on then turns on the burner. The flow switch measures the amount of water flowing and adjusts the size of the burner to the amount of water flow. So a couple different things might be happening. The flow switch might be sticking causing the water heater to throttle down the burner but not off when the water flow is off. Or the gas control valve might be by-passing gas when off causing the burner to stay on even when the demand is off. Run the hot water on high demand then turn the water off. If the burner is still on try banging on the water heater with your hand and see if the burner shuts off completely. If it does the gas control valve might be by-passing. You would also need to check and see if the pressure/flow switch is giving any voltage signal changes when the water flow is off. These tankless water heaters are more comlplicated to work on than a standard water heater and you may need a professional's help...good luck

Thermo coupler may be defective. it controls the main gas valve by closing main if pilot is out. make sure thermo coupler bulb is positioned over and near tip of pilot flame, but not in the flame. Thermo couplers are inexpensive and easy to change out.

LP OR NAT GAS , IF IT IS LP CHECK GAS PRESSURE @ TANK AND MAKE SURE NOT TO HIGH , IF NAT STILL COULD BE THE PROBLEM HAVE THE GAS COMPANY CK PRESSURE

the diaphram is the pushing force for the gas valve to open, if you have a split or worn diaphram it will ned to be replaced.