Air switch troubleshooting checlist......
Plugged flue (bird nests, etc.)
improperly sized flue (see installation specs)
plugged pressure switch tube (remove from air switch, and inducer assembly to clear. then take paper clip and clear connection for tube to inducer assembly. replace tube and call for heat.)
lack of combustion air causing negative pressure in utility room (ensure utility room has proper access to cumbustion air required for make and mark of unit.)
improper low voltage readings for proper operation of air switch. (check electric diagram for proper votage, and see if the voltage at the air switch matches.)
improper voltage at inducer motor. (check diagram for proper voltage, check voltage at motor to see if matching.)
dirty inducer blower wheel. (rare occurance, but worth looking into last) Once I found a mouse that made his way all of the way down the flue into the inducer housing.
If all of these checks are passed, then you have a rare occurance of an air switch going bad.(replace it)