Been fighting this problem for over a year now...about to go crazy. Ok so at 129000 miles I blew my motor being a dumbass. So a complete rebuild was done including the heads as to have the new style valve seats installed and just so it was all done. And I have replaced cap, leak detection pump, egr ,plug wires , total throttle body cleaning and have smoke tested several times. After reset the code will stay away for 1 to 3 days then both return at the same time every time with an occasional P0456. Help!

 

P0457/456 are evap codes. The other is a random misfire. Chasing a random misfire will drive you crazy. If you or you know someone that has a scanner that can read the misfire counter it will give you a place to start.

 

Yes I do know the codes and has been scanned too many times now. The results are always random on the misfires. Having a hard time putting the two codes together......gas cap code leading to misfire??? And yes they appear together at same time always.

 

Two separate issues. No gas cap will not cause misfire. There is a different between a $20.00 code reader. And a factory scan tool. The factory scan tool can watch the misfire counter. It can also look at the functions on the 02 sensors. And may other items that can help.

 

Yes I hear you my truck is currently in a state of the art shop on a state of the art scan machine. The truck will have no codes and immediately upon throwing the P0457 it throws P0300. So in some strange way there has to be some relation.

 

Has it ever had a PCM re flash? May want to do one. I have never seem or heard of evap leak causing misfire.