Fuel mileage my grand am is a 1997 with a 2.4L everything for the ignition is new o2 sensor/tps is new have ran 2 bottoles of fuel injection cleaner throught it have 152675 miles on it only getting 15 miles per gallon of city driveing i have been told i should be getting at least 18 to 20 the o2 sensor has only been replaced a week ago before replaceing it i was only getting around 9 miles per gallon

SOURCE: cylinder 3 misfire

Make sure you're using the recommended spark plugs. When my car did that, I had put Bosch Platinum Plus-4 plugs in and got that same problem. Turns out the plugs are too hot and cause it to fire prematurely and kick the misfire code.

SOURCE: 1999 pontiac grand prix . will not run

If wiggling the wires cured the code, then one of the wires is broken inside the insulation. Pull on each wire one at a time. look for the colored insulation to stretch, this will be the broken wire. For the other car that wont rev up, this is a common when the catalytic concverter is plugged up. This will also cause MAF codes, and in turn, TRAC OFF light.
See http://www.greatautohelp.com

SOURCE: 1994 pontiac transport se sluggish running

check the sparkplug wires on the firewall side for arching-body seam there that will cut them

SOURCE: 1992 Pontiac Grand PrixPower loss,erratic idle

spark plug gaps are very critical on computerised cars!! [some wont even start!]
distributor module may be breeaking down as temp rises they fail!
otherwise do compression check as it may have valve problems
also check plug leads & injectors aswell as the fuel pump pressure [ could bestarving for fuel]

SOURCE: 04 Pontiac Vibe bad gas mileage without a code and then the p0420

that code is from having the plugs wires reversed as you did and the o2 senors regrsitered the overload on gas emmions once you remove the battery cables for about 5 minutes and then recconect and run the engine it should be okay