the furnace should not run if the IDM isnt working, if it does, there is a problem with a pressure switch that you need to locate, it should be up near the IDM. the IDM should have been packaged with a new gasket OR a tube of silicone. its very easy to do.
the IDM needs to close the pressure switch before the circuit board will let the ignitor start heating. the board could be bad too, if its letting the furnace run.