01 Mitsubishi galant v6 P0455

I got the P0455 code in Dec. Then I replaced the gas cap. Everything was fine till last week. The service engine soon light came on right at startup. I was in no hurry so I read the codes. Got P0455 and P0455 pd (pending). Shut car off cleared codes removed gas cap and reinstalled it being sure to go at least 3 clicks past stop (actually went 6 ). Started car everything fine. Now 3 days later the same code is back. I got my new cap at Auto Zone which some say was a mistake but it worked for this long. Do you think the gas cap is bad again or is something else the problem?
The P0455 code means Emission Control System Leak (large) according to the book that came with my reader.

 

You may have a failing EVAP canister. If you have replaced the cap, and now it's back, I would bet it's that.

You should use OEM gas cap as well. Autozone type caps are not the best of quality. Also inspect your fuel line hoses for leaks.