1997 mgm,4.6l,rear wheel dr.

heres a question.i"ve owned a 97 grand marquis for the last 2 and half yrs.bought it from a private dealer.right after i purchased car,the check engine light came on.called the dealer and explained to him what was going on.he said that it was probably gas cap,to go ahead and loosen it.while loosened,remove neg. side of battery cable to reset code.ok ,fine. did that and the light went out.drove car for about 20 miles and the light comes back on again.repeated process everytime for the next 6 times after that.car ran fine the whole time.after getting aggravated with it,left it alone until recently.had heater core hose rupture under intake manifold.being somewhat mechanically inclined ,i repaired it myself according to process in haynes shop manual.2 weeks ago,i broke down and purchased a scan/code reader and retrieved codes from pcm.codes were p0171 and p0174.t/s why codes were happening.after doing research online,i found majority of forums talking about dirty maf sensor.so,i removed and cleaned as recommended.while doing a visual inspection of engine for vac leaks, i discovered a pinhole in boot for egr.replaced as req.cleared all codes and ran thru several drive cycles.got car inspected and passed with flying colors.today driving down road,had light come on again.hooked up scan tool and got a p0430 code.researched what it was.NOW ,since my wife never watered our money tree this yr.,my wallet is very thin.if anyone has a pretty good diagnosis as to what caused this code and could save me some time and money,i would be much appreciative....really sorry about the long version of this story,but i figured with as much info as possible,maybe answers can be had............thanx,dano:runaround::banghead:

what would you lean to,the o2 sensors or just go for the cat.trying ot to puttouch mone into car.