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HI my name is Robert...
What is the make and model of the water heater??? Also when you try and run it on LP do you hear any noise at all?
Go to the outside access and locate the circuit board, pull the connector and then clean the contacts on the board with an eraser. Sometimes this is a weak link. IF that harness has voltage on it when you have the switch ON for LP or Electric it is most likely a problem with the board. The water heater should have a high limit for BOTH the LP and AC side so it would be rare for them both to fail. Normally the high limit for the LP side is on a brown wire, the wire would have a clear shield over it, that is a one time fuse, so if you see that make sure you have continuity through it or it does not look burnt.
It also could be the circuit board assuming you have voltage on the harness, most RV stores can test the board with a special tester and see if the board is at fault, they do fail. IF your unit is still under warranty I might consider having a shop take a look. IF you still need help please get me the model of the Atwood, should be on a sticker under the outside access panel. Hope this helps some......
IF you are satisfied with my efforts please feel free to select a rating at the top of the pafge, it is the only way I get credit for helping... Robert
HI.... Was just checking in to see if my answer helped???? IF you have more questions or concerns please feel free to contact me..... Robert