If you have the same setup as my DD 6V92TA, you will have three parts to this system. The electric air valve (skinner valve), the piston, the fuel shutoff lever. With no power to the skinner valve, it allows air to pass through, extending the piston, pushing the fuel shutoff lever, and thus shutting down the engine. Apply power and no air, no piston, lever released, engine starts.
So, in the past, I have had my piston get stuck in the extended position, not allowing the bus to start. Skinner was working.
Hopefully this will allow you to troubleshoot the system.