Gas has to be on for frig to work in automatic mode

make and model of fridge is helpful

This is an RV frig correct? It SHOULD work either/or. AC or DC. If your plugged into AC then often the frig will work with that alone. DC is for when you have no AC so you can power your appliances of the battery. Maybe you have a converter (AC to DC which most RV's do) that somehow is not getting the DC to the frig.

If you need further help, reach me via phone at https://www.6ya.com/expert/steve_24d988f3507f7480

The dometic is designed to run down the road in the gas mode, but NEVER fill you LP tank with you unit switched on, in either the gas or AC mode. Make sure you shut everything down before filling the tank!

http://67.238.126.140/pdf/3309661.001.pdf

Dometic Voluntarily Expands Refrigerator Recall

Dometic recently expanded its recall on a population of two-door refrigerators. The original recall included some two-door refrigerators manufactured between April 1997 and May 2003. The new recall focuses on models manufactured between June 2003 and September 2006.

To see if your unit is on the recall list & what to do, try the link below.
If your unit is on the list, DO NOT run your refrigerator on propane until the kit is installed

http://www2.dometicusa.com/recall.php

Auto mode is electric 1st then gas. If no 110v is available it tries to light the gas. Do you have gas & is it turned on? You should hear a clicking from the back of the frig. Make sure the air is purged out of the system by lighting your range and let the burners come on for 30 sec or so. You might have to cycle the frig on & off a couple of times after you purge the air. Set the button to "gas only".

This one is easy your thermostat is gone. You may notice the unit never shuts off.

Youneed a new sealed system, you have lost gas. ( Smells of amonia)_

If the battery voltage is below 10 volts the fridge will not operate. Check your burner for rust.