if it is posible to take off the old cat, clean out all the guts then reinstall it , ask around if that is posible

 

You have multiple choices. You can buy a Nissan cat for about $500, an after market bolt-in for around $300(I think), an after market generic cat for about $75, replace your cat with a straight pipe for maybe $15, or gut you cat for free.
The cheapest & most legal option would be to get the after market generic one, take it to a shop & get it welded on & be back on the road legally for maybe $150-200.

 

Thanks any certain after market cat that you would recommend? I also am wondering about how much more fuel economy I may possible gain. Right now I get only about 270 with about 2 gallons left over. Also how many cats are on frontiers i have read other forms that some truck may possible have 4 on them

Would this one work if i brought it to a muffler shop and had them weld it on

 

Wouldn't a code still show if you replace the cat with a "test pipe"?
A CEL and a visual inspection is only problem in Craptifornia anyway.

 

Could those codes be coming up faulty because the cel comes on for 3 weeks than of for like 3 weeks give or take

 

Your cat is on the verge of going bad or the downstream sensor is going bad...I would have it checked out by a tech first but If you gut it out the light will still come on, not to mention is it illegal in most states and carries a hefty fine. If you have any kind of emission inspection with obd2 it will fail because of the CEL light is on. What year is your truck? If it's 8 yrs or under 80K your still in Federal emission warranty with Nissan and they will replace it free of charge.

In my opinion, get the one from Nissan, I know it's more money but you will save in the long one because the aftermarket ones don't last as long and Nissan's are verry touchy and aftermarket can turn on the CEL even though the cat is new.