My "Service Engine Soon" light came on a couple weeks ago, I just broke 60,000 miles so I thought it might be configured to come on, but the car has started to idle while stopped it gets pretty rough. It also seems to fluctuate while driving, takes more effort to get it to speed up, etc.
I got an oil change and then took to Auto Zone for the codes which are:
I already replaced the gas cap a few months back....
The codes PO440 and PO442 are for the EVAP (Evaporative Emission Control System), the system controls the collection of gas vapors and stores them in a carbon canister so they can't escape into the atmosphere until they can be burned in a proper controlled condition.
The codes are telling you that there is a leak in the system. It could be a cracked or eroded hose or line for the EVAP system anywhere from the gas tank, carbon canister and/or the vacuum lines from the engine to the EVAP system.
They have a new machine that is hooked up to the EVAP system and fills it with harmless smoke. The mechanic looks for the smoke leaking from the system which identifies the location of the leak for the mechanic.
Did you get a new gas cap because of the check engine light being on or was it replaced for another reason? It seems that you changed the cap and soon after the light went on.
Bring your truck to a qualified mechanic that has this smoke machine which saves a lot of diagnostic time and money and he can also check the fuel cap for proper operation.
Let us know how you make out. If more diagnosing is needed we will be glad to help.