How do I remove the thermocouple from a Coleman 7900 seris Presidential furnace? The pilot stays lit only as long as I have the button depressed.

Removal of a thermocouple is a mechanical type operation. One end will screw/thread off the valve. The other end will pull out or have a clip that holds it in place. If you are working on a Mobile home then you will have to pull the burner out. That consists of turning off and disconnecting the gas line and removal of attaching bolts/nuts that hold the burner in place.

Many times it is better to contact someone local if you do not know what you are doing on a mobile home furnace since they are a bit more involved that a stick built home furnace.