I have a Ruud 50,000 BTU furnace and the furnace lights but the flame goes out about ten seconds after burners kick on. The yellow light never lights up during entire process. The green light is always on and does not blink. I cleaned the flame sensor with fine sandpaper and re-installed after surface was shiny. It seems this is a gas valve problem but not sure. Hope someone can help with this problem.

Are you sure all burners are lighting and the flame is hitting the flame sensor? Make sure the flame sensor is not touching the burners or any other metal. If the flame sensor or sensor wire grounds out it will not work.

Yes, all burners are lit and flame sensor does not touch any metal including burners. The yellow light never comes on after burners light. This should happen. Green light is always on. I noticed that the yellow light flicks on only for a moment when turning off or on the power to the furnace. I wonder if I have a ground problem in the power disconnect when panel is removed. Furnace will operate with panel off as well. This should not be.

The power disconnect usually is just the hot wire. I would carefully check the ground and neutral connections. Might be a good idea to find and fix the problem with the power disconnect too. Safety switches are a pain at times, but save lives.

You do have to have a good equipment ground for the control board and gas valve. The gas valve usually has a ground wire going to the manifold at the burners. If this wire is broke, missing or burnt into the flame sensor will not work.