So, 2005 Nissan Titan SE Crew Cab short bed. 5.6 v8. Big Tow series.

So purchased the truck used. From day one, It had massive redflags. Scotty I know you used to live in my home state. New hampshire. As you are aware, In N.H. If purchased from a used car dealer. If you finance they must pass the car if for safety and give a 20 day sticker. So I signed the paperwork. An I take the truck. This was a few years ago.. Might add.. about 7 yrs ago now. Truck had right at 100k for mileage. I agreed to 11k sales price. i put down 3 grand and financed 8k. Ill explain the finance part in a second in case your wondering why I am mentioned that part. So He says he cant get me into the garage to be inspected til monday. Its sat. So I agree to return on Monday. Going home. 20 minutes in and 2 miles from home. MIL comes on. I call the dealer and he says he didnt flash the ECU but we can address this on monday when I return. I turn from home and head straight to advanced. Not to buy a damn thing, Even though I have a obd2 reader. I want a third party to run a diagnoistics on the ECU and get a report, that has the date on it. An the codes listed in print. So I have my ass covered.. Got a p0420 code.. but we all know. the damn things can go off with very little obstruction.. even at 94% it will trigger, cause 5 percent is all it needs. So on monday. The fun starts.. Truck fails. due to 0420. I argue no way in hell am I paying 11k. for a truck that wont pass inspection. Mind you I never signed the paperwork to buy, only a promise to buy to take the truck til monday. So he tells me he will fix it.. and i leave it there. Now mind you. on Sat. He assigned me a 20 day plate. Each day he has the truck. Is a day off the 20 I have before I register. The next 20 days I call every day and he says its not ready. Finally on the 20th day I drive there.. last day of the tag. The truck does not have a sticker. He says return at 430 and he promises to make it up for me for having the truck for 20 days. and it will be stickered. I return and damn it.. it did have a sticker. so He gives me 200 for my trouble.. and I leave with the truck.. 20 minutes.. with me running the drive cycle on purpose.. MIL ENGAGED... I run obd2. 0420.. result.. I realize.. this truck has had nothing done too it, besides a fuel treat ment but how did it run? So I look at the inspection sticker. I do some research and find the name of the garage that inspected.. so I go to that garage.. and come to find out.. yep they inspected a truck for the dealer.. But it was not mine.. Dealer brought a dodge ram there.. and peeled the sticker off.. and put it on mine after.. I had him dead to rights.. In NH. Inspections are overseen by the NH STATE Troopers.. I return to the dealership.. And I confront the dealer and the owner. I demand that I am released.. from this.. I will sue.. and they will be answering questions with the troopers.. shortly after I leave there. if they refused. The owner. Had no idea.. this had happened. and to be honest.. he says anyways.. he would be the one on the hook. HE instead offers to give me the truck.. At a extreme discount. He purchased it at a auto auction for 3700.. remember 11k .. damn kiss me first/?? So I own the truck now for 3700.. and he fired the salesmen on the spot. But at 3700.. he also fixes the issue. I counter with. 3200.. and you dont fix anything. I take it to my own mechanics. he agreed. An Now I own the truck. So I took it to Nissan and had them do a 125 full inspection for issues and report. Comes back with the 0420.. Being caused by a bad A/F sensor.. in bank 1 caused by it being changed but their was a service bulletin that the ecu will not relearn without the use of a special scanner used by nissan. It cant be flashed to relearn unless you have the exact program for it.. and it cant do a auto relearn by unplugging the mass air flow sensor. So after that. they place the 125 I paid.. and put it too the cost of the relearn.. and I pay 125 for the inspection and the relearn. after that.. I get a code 1273.. which was a crack in the 02 sensor on bank 1.. Which I sealed.. and welded a new bung on and codes 0420 and 1273 are now both gone.. for a few weeks. Now fast forward. I have a code 0420.. now currently. I have checked the temp before and after the cats.. both on bank 1 and 2.. so all 4 cats.. temps are in norm range. I used a compression test.. and while the poundage I got I was not very happy with.. it was right in the normal range.. but it could have been better. 1.8 for back pressure is what I think is right at the normal range. Anything higher is bad.. Im at 1.9. My question is, Do you think My cat may actually be clogged? or could it be another cause for the 420 like before. Also , I have ran Cataclean, and guar to pass.. not together but in different full tanks. I have changed all 8 spark plugs, and also checked the fuel pressure. I have also replaced the fuel filter. I just dont wanna replace the cat before I have anything else that could be the cause. CATS on this truck are expensive. Could it be a cracked exhaust manifold, I hear no ticking cold. Or a small exhaust leak in the pipes or flanges.. I have tested on cold pipes with postive air running in with soapy water. I am at a loss and ready to bow my head and buy the universal cats and install them.. I will have to pay for this as I suck at welding. Any thoughts? Thanks.