'97 contour OBD II codes
 

I have hard time troubleshooting, can do repairs ok. Have '97 Ford Conour 2.5L a/t, a/c.
Have Total of 8 codes, I know what they are but don't know how to troubleshoot. Codes are, PO420, PO430, PO171, PO174, PO300, PO305, PO306. WOW!!!!

Had A-1 Exhaust check catalytic convertors, says they are ok, they DID NOT hook up to machine, I thought that was a poor way to check. Didn't cost anything, they only revved eng. & said it seemed like there was no blockage. Guess I should've taken to a dealer to have that test done, can't afford to pay Dealership costs.

I welcome all tips & suggestions.