Ok, Tankless Water heaters are typically Propane only, However they may (Should in fact) be using 12 volt electrical for several function.. Like opening the gas valve and lighting the flame.
Here is an opration description for a 12 volt controlled Tankless DSI water heater
Water flows, closing a flow switch (like a sail switch on a furnace, in fact, other than being waterproof, it is identical)
IF thermostat is cool, then the control board opens gas vale and inititates sparking (ignition)
Flame sensor sends "I'm hot" signal back to control board and flame continues tilll...
Thermostat says I"m hot or water stops flowing.. (either one).
Possible issues:
Water flow too slow
Switch (flow) bad
Thermostat bad
Mother board bad
No propane
Propane valve bad
Ignition points set too far apart (Very popular error Half the DSI products I've worked on had this). (two out of 4)
Bad flame sensor
Bad control board again (Flame detector circuit)
And of course, no 12 volts (blown fuse, bad connection, broken connection, Switch OFF)