What causes hot water heater Fault light to light?
Well, if you already have HOT water you don't need a HOT WATER HEATER.
But seriously, that light tells you the water heater failed to light and is not heating the water after several attempts to light. It will not try anymore until you turn it off and then back on to recycle the controller.
To amplify a bit….when you try to use the heater on propane the controller goes through a start sequence. It turns on the gas flow and the last step is “sparking” the igniter. You should be able to hear it spark rapidly. If it fails to light the controller will shut it down and turn on the light so that you’ll know it failed. As mentioned you can turn the switch off and back on to restart the process. It’s not unusual to have to do that two or three times if you haven’t used the coach for a while. Usually it just means there is/was air in the line that has to be flushed out. Many experienced RVers will light the stove first to ensure there is good gas flow.