bad idea - disconnecting PVC air inlet
First of all, I'm a homeowner not a pro.
A certain amount of resistance is expected in the PVC pipe inlet, and the inducer fan is adjusting its speed according to this, so:
- piping should be right size (varies accoriding to distance, altitude, and furnace size as specified in the installation manual tables)
- check for the "perforated disk" on the air inlet at the combustion chamber air inlet, which should be installed in most cases
Neither of the above are likely candidates for the 42 fault, but I wanted to mention it.
For your own info, download a trouble shooting guide (check your furnace series number to see which one is applicable). It's not a do-it-yourself repair or diagnosis suggestion - just helps to know as much as possible.
Here's a pointer (main website is http://www.commercial.carrier.com - select the literature link)