A buddy of mine has been battling problems with a furnace not lighting and has figured a way around it (safely) and a repairman has been out 3 times but still not 100% fixed. It's a Payne 74k BTU furnace from 2002, not sure of the exact model. It is a condensing style furnace. What happens is he will turn it on from the thermostat, starts up and works fine. On the next cycle it will not light, tries three times, then won't try anymore. He can turn the furnace off and restart it and works fine for one cycle, then back to the same problem. The service technician has replaced the ignitor and cleaned the flame detector. My buddy has vacuumed the condensate lines and has made sure the P trap is full of water. Voltages from the circuit board were also checked. Based on some research my buddy thought maybe there wasn't enough air for combustion and removed a side panel and now the furnace starts up every cycle. The flame burns a lot stronger and air coming out of the vents is warmer. On the last trip from the technician they both inspected the air inlet pipe from outside the house and couldn't find any blockages. He thinks that maybe the inlet pipe is undersized, which is 2". I found a sizing chart for another brand and 2" seems to be plenty big for a 74k BTU unit. Any ideas on what the problem is? My only thoughts are either there is still a partially blocked inlet pipe or maybe the blower motor is a little weak.