Tankless water heater pilot light will not stay on

sounds like pressure relief valve is stuck closed free of or replace

Before calling service technician, copy following links for troubleshoot resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

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Tankless should never be installed with plastic pipe, PVC or CPVC etc.

Before calling service technician, open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual.

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Insufficient gas presure. Check gas presure exiting regulator exrternal of water heater. If presure is at 3.7-inches water column gas valve is defective.

Check and be sure gas knob is in the pilot lighting position and pushed in. Try to manually light with a match. Make sure gas **** is in the on position. Posible air in the gas line. Bled off air. gas valve or regulator has failed..

there is a small dia copper pipe which has a thermal coupling on it the end of it sits in the flame of the pilot light,
if it fails which it does after about 25-15 months,it shuts down the gas burner

Check to make sure the flame of the pilot is hitting the thermocoupling. If not then you can use compressed air to clean the pilot port and relight and check again. If the flame is hitting the thermocouple, then it may need to be replaced.

OVERHEAT STAT OR FAN

it will tell you right on the box if it for indoors or out doors either way they have to have a cover over them,,

sounds like your mixing valve or tempering valve not unit.Somewhere there is a unit that prevents scalding temperatures- I expect this is your issue find and adjust this first- there are directions on it when you find it.