[Michael_M]

DTC stands for Diagnostic trouble code which is the same as the CEL.. Sorry but I got used to say DTC other than CEL as DTC is more common!

CEL/DTC 254 Large Volume air leak that might be triggered from just the fuel cap.. It might be leaking vacuum causing disturbance in the fuel system and thus causing the misfire and hassling.

DTC or CEL 62 then you also might need to check the whole Secondary air pump system.. It doesn't really means that you need to change parts but might be a simple short circuit causing that issue which is labour intensive to run after each and every wire and try to know what is causing all that!

Sorry I thought it was throwing different codes every time that what made me think it won't be related to a specific system but then since it throws the same code every time then it must be something either affecting both systems or affect one system that affect the other.

Did it start to act up like that by itself or have you replaced or fixed something lately and then that issue started to happen?

My Suggestion is to replace the Fuel cap as it is the cheapest option, go for a run and see how things will go..

If the ASC is always on check Fuse no. 21(5A) and fuse no. 38(30A). ASC control or have control on the TPS (throttle position sensor), a power surge might happen triggering the ASC Control Module.

Sorry I know it is confusing but electrical diagnosis are way more hectic than mechanical diagnosis.

I have a feeling that is caused from a short circuit or corroded wire!