I have Goodman Furnace model GMT 090- 3 installed in 1999 and it has stop working and when I open the cover of furnace I can see there is a red LED light that blinks three times and stops after a second or so it again blinks three times and stops this cycle is keep going, and furnace doesn't start. The diagnostics info on the panel says it's a pressure switch and to check the 'venter' for a blockage. I'm the 2nd owner and don't have a manual. What and where is the vinter and how does one check it or fix it? Thanks