I have a goodman furnance#GDT070-3B installed in 2003.until yesterday it would only get hot for 5 or 10 minutes then shut off heat while the blower kept running.repairman replaced the pressure switch twice,a different switch the third time an now says it's the control board.the board has always flashed 3 blinks during each repair.last night after reading comments here I cleaned the flame sensor,Cleaned all dirt & dust off fan blades,cleaned the vent tubes vecumed all dirt&dust inside the furnace.I even took off the control board&blew it off with air.now it won't ignite,only the blower runs and the board starts blinking 3 flashes as soon as the power comes on.What did I do wrong?I need heat now.PS I`ve alredy paid almost $400 for Repairmen to not fix the problem

With a 90% furnace if it can not drain the condensation the pressure switch will open. Check for a restricted drain inside and outside of the furnace. A slight restriction in the air intake or exhaust will give the same.

My furnace doesn't have a drain.