Q: valve cover replacement, chevy cruze LS 2011, the light was on, but then I don't see it anymore

What exactly was wrong with the "valve cover"? Valve covers are not typically replaced unless the cover is cracked or deformed. On cars, the check engine light can, occasionally, come on, stay awhile and then disappear never to be seen again. "Glitches" happen. HOWEVER, in your circumstance, inasmuch as you mention a tangible symptom, that is "shaking", you may have what we call an incipient failure that starts out "intermittent" but eventually may become obvious. The thing to do is this: If any warning light returns, check engine related or otherwise, simply request a check engine light diagnostic during which a certified mechanic from YourMechanic would use a code scanner to retrieve the diagnostic trouble code(s) from your car’s PCM (that is, the specific codes that caused the light to illuminate) and then test the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like) to PINPOINT the reason why the check engine light illuminated. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.