Closing in on 100k and CEL came up.

Ok, so I'm closing in on 100K and my CEL came on. Had a buddy check for codes and a few came up (P0113, P0303, and P0421). Intake temp sensor, cylinder No. 3 misfire and Right bank Cat system efficiency below threshhold. Pk the 1st two I sorta understand, but the 3rd I have no idea what that means. I'm thinking that the temp sensor might have been affected by the K&N filter I had in there (really dirty), and needing to change my plug wires for the misfire. N E help is appreciated.

the 3rd one means that the you need a new cat, or sometype of replacement. dont bother going to the dealer for that....

except, there isnt a right bank on the cars...theres front or rear cat...and im gonna guess its the front, cause they normally go first.

I'm new to the forum; I have a 2001 Protege which I absolutely adore. Just turned over 60,000 and never a problem. My check engine light came on yesterday and I took it to AutoZone...possible causes showed as EVAP canister or hose cracked; purge or vent solenoid defective; or vacuum leak at engine. And of course, the very popular loose fuel cap possibility.
I'll admit up front to being a "girly" girl. What is my wisest course of action here, now that I've tightened the gas cap and the light still hasn't gone off?

ok frist to Antixpro before you go and get a net cat look into a header it deletes the first cat and if you do the non-fouler trick you won't have a cel problem now make sure you can remove the cat because of some state laws
and now onto Librarycat welcome to the forum
to get rid of you cel just disconect the positive side of the battery for like 10mins and that should reset your computer and take away the cel light

Ok some updates. Since last we met....had my techs at my dealership clear the CEL about a month or so ago. Yesterday CEL came back. Took it back to my old friends at the Mazda dealership and they say I need a new pre-cat. $340 in parts and $160 labor. Car is paid off, but I still need to get it smoged for registration. Any ideas on what I can or should do?