If plugged in, and you're on "Auto" you should be on electric. If you flip manually to "gas" it should run on gas even if you're still plugged into shore power.
If you're on "Auto" and plugged in, you're on Electric. If you loose shore power or unplug, the refer should automatically switch to gas, unless you are already on gas, then nothing will change for the refrigerator.
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Like everybody already said, just don't freak out if you don't hear it fire up right away. If the fridge is already cold then it won't need to "work" as they can stay cool for a while, so it might be a good idea to do a test when the fridge isn't at it's coolest.
Could give you a certain answer if I knew what kind of fridge you have (I'm pretty sure Puma doesn't make one) but with MOST of them when it's used correctly it will automatically switch. Of course you MUST have LP in the bottles/tanks, and the valves must be open and the fridge MUST be able to run in LP mode (clean burners, good spark unit or pilot light lit) and everything in proper working order.
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