I have an on demand water heater. I have 3x 40 amp breakers in sub pannel as that is what it calls for, but only a 100 amp in main panel feeding sub panel. Would this cause no hot water? The lights are on but no hot water.
No, the 100 Amp breaker feeding the sub-panel is not the reason for the water heater not working.
Has the heater ever made hot water, has this started recently?
The 100 Amp feed is wrong for other reasons
Others will likely talk about 'code violations,'but without knowing when and where your system was installed, those are just guesses at this time.
If the lights are on, and the buttons and dials work correctly - we will take a leap of faith and say the electronics that are not directly connected to the heat elements are working.
I am also going to suppose that the heater is wired correctly.
Also that the hot and cold water pipes are connected to the rest of the plumbing correctly, and that the needed valves are open.
So that when you turn on the water faucet in a sink the cold water to the house flows into the cold water port on the water heater, then flows out of the hot port on the heater, and comes out the sink.
This leaves at least:
power wiring to and from the connection points for the heater
flow sensor - 1
temperature sensor cold side - 1
temperature sensor hot side - 1
heating elements - 3
high temperature cut off switch - 3
scr used to control the cycling of the heating element - 3
If only 1 set of scr, high temp, and heating element work, then some hot water will be produced.
--- I will work on this some more, within the next few hours.
Electronics that are connected to the heating elements