3 way fridge?

my three way fridge works ok on 12 volt and 240 volt but when I connect to gas I cannot get it to work. (1) does it have to be connected to the electric? only I have disconnected it from mains and 12volt. It has a full gas tank, and its switched on, I hold down the gas knob for 15 seconds and press the electric knob as instructed but nothing
It has not been on gas for quite a while now and I was wondering if It may have to be bled ?? any ideas??