Model M1MB07A BW Intertherm gas furnace. Furnace will not come on automatically. Only works when reset button is pushed. What could the problem be?

To which reset button are you referring?

Furnace has on/off swith on top of the control module. When furnace isn't operating, red light in module flashes twice indicating "pressure switch is open." when I turn on/off switch off for a few seconds, then turn back to "on", red light stays constant indicating "control ok". After a minute or so, yellow light comes on indicating "flame is sensed." After another minute or so, the fan comes on and the furnace operates properly. This seems to be the only way to get the furnace to turn on. What could be wrong?