P0421 Mitsubishi - eclips

Question

I am getting the P0421 code along with P0152 and P0130 (when warm). This is a 2007 Mitsubishi Eclipse.

I had this checked because the check engine light would come on from time to time. There is no power loss or any engine running issues. There is no black exhaust and fuel economy is normal. We did notice that if we played with the gas cap after the check engine light came on (3 times) the light would go off). It seems after prolonged driving the gas cap would loosen. The mechanic said these codes had nothing to do with the gas cap (no evap codes). They want to charge us over $1200 to replace the front catalytic conv.

Is it possible with these codes we just need to replace the gas cap?

John