Will a loose gas cap cause the check engine light to come on. My CEL displayed shortly after our return trip from a vacation however upon my next gas fill up I noticed that my wife did not have the gas cap on tightly. After securing the gas cap after filling up the CEL still remained lite. I took to mechanic and they indicated that New Evap Vent Valve and Purge Valve needed to be replaced. My vehicle has only 62,000 miles.
Loose gas cap. on 2003 Pontiac Montana
If the gas cap was loose, then that is likely what caused the light to come on. It's unlikely that both valves failed at the same time the gas cap was left loose. Have the shop note down the codes that were stored, have the codes reset, and if it comes back on, take it to a different shop if possible.