Aq 125 blp Bosch tankless water heater, pilot light has a weak small flame, goes out easily. HOW do i increase the pilot light flame on this model?

Most of these have a safety switch, that would make the pilot shut down. Also check for rust deposits in your fuel line, and pressure.

Remove the pilot injector from old unit and fit into new unit

Clean your orfice...it sits at the bottom of your pilot burner, where the brass pilot tube screws in to. Its a small stamped piece of stainless, about the size of a pencil eraser. You can also buy new ones online for only a couple bucks

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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For a price, expert works with you while you work on water heater or any do-it-yourself project.
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It sounds to me like your thermocouple is bad . See if you can trace it from the gas valve to pilot light. Remove thermocouple and try to get a exact replacement part. If the length varies don't worry about that. Install new thermocouple and see if that doesn't solve your problem. Thanks

The pilot light can be blown out just by the main gas being ignited especially if the pilot flame is a little weak. Some times a bit of grit gets into the pilot nozzle reducing the flow and hence a weak flame. Try cleaning the piolt jet nozzle with a fine piece of wire. You may have to dismantle the jet nozzle to clean it properly.

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.

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,,

the pilot is getting too much air and is an indicaiot of dirty or air/ fuel ratio imbalance. The pilot is wimpy and the thermocoupler has alot to do with the gas . Make sure your thermocoupler is on the tip of the flame. You may end up replacing it. because it is showing signs of eminnt failure. When it fals your curners won't fire up. Good luck.