[beenthere]

Where are the thermostats? Or do they use duct stats? What have you checked so far?

 

[beenthere]

Somewhere you will have your pneumatic control panel. Check to see if one or more of the transducers are bad. One of the pneumatic stats may be bad and keeping the unit on.

Those stats are most likely reverse acting, so you may still have an air problem. The control panel should have the pressures you stats work at, make sure they are getting that pressure, and that it is coming back to the transducers/actuators.