Code light flashes 3 times... I can jump out the pressure switch and get the system to run for one cycle.
Took out the pressure switch and can make it work by blowing into the tube connection but it does not close when the induction fan comes on. How do I check the pressure switch for proper operation?

More then likely the psi switch is fine. If you have the wires connected to the psi switch and you create a positive psi using your mouth and the ignitor warms up, you know that the diaphragm inside the psi switch is working properly. Withought confusing you well start with some basics, take the hose off the psi switch and blow the other way, make sure that the lines are clear, if it has a "t" disconnect the "t" and blow threw each hose. The next step would to remove your condensate hose and blow threw it. On the bottom half of the furnace cabinet on either the far left or far right depending on how your furnace drains you have what they call a condensate trap trane's are usually white in collor, if you can look at it and understand how it comes apart withought breaking it, remove the condensate trap and run hot water with a little soap threw it blow threw all the lines connected to it and make sure that they are clear. If it still doesn't work walk outside and check the intake and exhaust, if you can't see anything and you still haven't got your furnace to work message me back and ill explain how to check the intake and exhaust.