2000 GMC Sierra, engine light came on today

Driving from work today the engine light came on, it is 2000 GMC Sierra, out of warranty with almost 50 000 km. I got my OBD-2 software going, read one code, P0442 Evaporative emission
control system, small leak detected. I was bit puzzled where to look for a problem, opened and retighten my gas cap, just to convince myself that it was tight. Opened the hood looked around, not much to see, I was looking for broken hose or something. Cleared the code, run the truck for a while no trouble code set again. Will try it tomorrow on the way to work, at highway speed. I think the engine light was set today at highway speed, at least when I notices, if that makes any difference. Anyone would have any experience on that and could give me a hint or two? Thanks in advance,
Ziggie
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out
Will
Replace your gas cap.
Doc

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The followup
No codes set today
As the mather of fact I got locking gas cap shortly after I bought it, 3 and half years no problem. I still have original one, so if problem comes back I will use that one
Ziggie

gas
two?
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Just wondering how the cap helps emissions and/or why the ECM should care? I would think if anything that should be an idiot light "ya forgot it moron!"
~KJ~

gas
two?
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Sierra,
emission
broken
I
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