losing gas mileage

Ok guys, my 06 tacoma has 105,000 miles on it and recently started losing gas mileage pretty good on it. I went from bout 375-400 a tank to 300. I changed my air filter and got a k & n and also changed the plugs in it. Figuring it would go up but hasnt happened so far. Before I changed those things the check engine light came on and said cat below threshold effiencey and I figured changing the plugs would fixed that since I was burning rich. Well Check engine light came on again the other day and im not sure if my cc are getting stopped up, which seems odd for it to be so new still. Anyone else had this problem and what fixed it, really do not want to change the cat converter and dont want to straight pipe due to check engine light coming on all time.

 

Can you unbolt the cats to see if they are clogged?

 

Other common things to do are clean the maf and throttle body, make sure O2 sensors are good or replace them (cheaper than a cat)

 

Thank you for the tips I have the same problem