I have a Trane XE90 that is about 15 years old. I started having an issue that troubleshooting led to a possible bad pressure switch. I replaced the pressure switch, but still have the same issue. I replaced the draft inducing motor, and still the same results. I have verified my exhaust and intake tubes to be clear. The unit will start a cycle, the draft motor will run, but the ignitor will not heat up unless I give the process about 3 seconds and then place a jumper across the contacts on the pressure switch. At this point the ignitor will heat and the unit will complete a normal cycle. I have verified that there is good air flow through the intake and exhaust tubes while the draft motor is running. I have cleaned the tubing and orifice from the burn chamber down to the gas valve and the pressure switch, I have also drawn air across the tubing and verified the pressure switch operation with an ohm meter. It is still as if the unit is not making enough air pressure or has some sort of leak on the draft circuit. Any ideas before I call in a pro to look it over?