Didn't know I was missing a hose from air filter box because my truck ran great

What is your question?

And always passes smog had the hose put on and truck started losing gas mileage, had less power, gas smell, then 10 months later check engine light code p0420. Took hose off about a month ago and even though the air filter needs to be changed its already getting better mileage and the check engine light is off now. Did the hose being off cause the cat to go bad? (even tho it ran great and passed smog 3 times) and what did putting the hose back on (for the last ten months) do? or just coincidence and the check engine light would v'e come on eventually? and the fact that it isn't on now? I need to have it smogged and not sure what to do. This is a 2000 4runner sr5 4x4 v6 with 120k miles (105k when i bought it) iv'e owned for 4 yrs with no problems except had the starter replaced 1 yr ago. it doesn't leak / lose coolant or oil and the oil is very clean!!!