93% gas furnace board blinks 3 times

[joe dude]

i have a up flow gas furnace luxaire a g9t and the board blinks 3 times wich is the s safty switch if i pull hose from switch witch is to the draft motor and suck in to it the switch makes and the glow plug is activated and the unit starts. but when i put the hose back nothing . if i take hose from the switch that goed to the combustion chamberand pull off the fan pulls switch in and unit will start but as soon as i plug it back to the combustion chamber the unit goes off what is the problem also there is the drian for the unit and a little black box wich i think is the water trap there seems to still be water in it could that be cause there to be not a lot of suck and the fan. please give advise

[HVAC Doc]

Check the main condensate drain for blockage and make sure it drains freely. I would also check the inducer motor and air intake/exhaust pipes for blockages/obstructions. If you do not find any problems in these areas, I would have the inducer checked with a magnahelic guage to determine if it is running the proper RPM's and pulling the specified amount of W.C. (water column) to activate the pressure switch and have the secondary checked for sooting/blockages.