I have a bosch aquastar aq 125fx probably 9 to 10 years old it heats fine but after about year 2 it stopped working when you fire it up shortly or immediately after firing it up before. in other words it works fine if you run it once continually but if you shut it off you usually have to wait 1 to 2 minutes to fire it again. When you turn it on the second time you can hear the piezo electric spark ticking but no ignition. wait for anothe r1 to 2 min. and away it goes. any ideas how to make it fire every time? it is pilotless and always works the same way.

SOURCE: Looking for Outdoor Tankless propane water heaters

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,,

SOURCE: 125FX (with FXhood) turns off while in use

may be overheat stat or fan check little hose at fan area for pinhole best of luck

SOURCE: I have a Bosch AquaStar AQ-125-BNG tankless water

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

SOURCE: Pilot light goes out randomly on 6 year old Bosch

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.

SOURCE: I have a Bosch Aquastar 125ng with the water

i have an aqua star 125 ng an the pilot lights and stays lit but the main burners wont fire up when i turn the water on i have good water pressure so i kind of stumped and i just rebuilt the diaphram in it also