p0171 & p0174 FIXED WOOOHOO. o wait.. p0420?

Since my brother bought the car 6 years ago (2000 Nissan Sentra GXE 70k miles) it has had p0171 & p0174. The mechanic got about $2k out of him to fix it, but he was never able to for what ever reason. Starting last week I decided I would fix that god damn engine light. I ran a bunch of sea foam through the brake booster line. (no real reason except for the smoke show) Some restore through the gas tank, and some premium gas. I also hosed down the MAF sensor with MAF sensor cleaner. You're probably thinking way to go, I bet it is working tip top now. Well, yes and no. The sea foam got rid of p0174 ? I know it was the action of Sea foaming because I removed the codes 20 times to no avail, reset to computer as well, and than I sea foamed it, and p0174 was gone. So , being stuck with p0171 I cleaned the MAF and beautiful. You can probably imagine my pride at fixing this problem plaguing my vehicle for 50k miles being utterly destroyed when the god damn SES light pops up. I pulled over, and got my handy code reader out of my glove box (you never know when you'll need it!) and code p0420 pops up. Strange right? It is just like the other code with power, as in a minimal to no loss of power, the only difference is when it was running lean I was capable of getting 38.3 miles per gallon on average (and I drive about 200 miles a day), but now I struggle to get 31.8:(. Another piece of info is that when I start it, it idles at about 1.5k rpms, and when I put it in reverse, and then park again, it idles at 700-900 (about). Any thoughts would be appreciated. The lantern goes on about every 500 miles, but the code is usually pending, and 9/10 times the code goes away after a few days (600- 1k miles) To me it reminds me of when headphones start going bad, and you have to fiddle with the end piece to make it play in both ears, but then you don't have to fiddle with it the next day. Well anyways, thanks for reading so much, and any help is appreciated.

If you heard a back fire, using Seafoam , the catalytic converter is probably damaged (P0420).