How Furnace Gas Valves Work and Leak 

Make sure there are no spider webs in the burner tubes. You can clean them with compressed air.

probably a leak on a pipe. try to sniff it out or use soap bubbles.

@@NathansHVAC we pressurized the line and found a leak on a yellow flex line connection ! Thanks for the response !

It isn't a good idea to service a safety device that has to always fail safe. This could lead to jumping out a safety feature that could result in a fail to unsafe condition. A perfect example would be removing a temperature limit. Or, putting a spring in the wrong prosition in a gas valve.

@@NathansHVAC once you know how it works, there should be no excuse not to repair it. I know it is not financially favorable to the contractors but to the owners , yes!