[Pressure Switch]

I have a York unit for about 8 years now. There were a few problems from time to time, but it all seems to surround the Code 3 (red light flash 3 times). According to the menu, this could be pressure switch stay on 'open' position. Here's what I tested and done:
1. Replaced the inducer; it is noisy anyway, so I put on a new one; seem to work fine.
2. Cleaned all the hoses connected to the pressure switch.
3. Inspected the inlet and exhaust pipes, they all seems to be clean.
4. Replaced with new air filters.
5. When turning on the unit, the inducer will start and green light will stay on for a few seconds, and then red 3. If I measure the two outlet on the pressure switch, it will show no electrical resistivity or current ( relative to ground) before the unit is turned on. Once unit is on, the two outlet will show they are closed with no resistivity, and the voltage to the ground is about 27 volts. I think this shows the pressure switch is fine.
Can someone help to diagnos what could be the problem? Or, I should call for a specialist.

By the way, visual inspection of the electrical board seems to be fine.