The problem has been diagnosed and solved, at least for the past two engine/trip cycles. I bought a Craftsman CanOBD2 model #87702 Diagnostic Tool.. normally 124.99, on sale for 99.99 with an extra 20.00 off using a Gift Card in my wallet. It came with a USB cable and PC software.
The DTC or code was P1441, "EVAP System Flow During Non-Purge." I recorded the code and reset the warning light. It hasn't come back on thus far. For 80-bucks, the red and black Craftsman gadget is well worth it. The dealer would have charge min 50-bucks to diagnose.
Now for the solution. The GM shop manual says that the solenoid-operated Purge Valve could be leaking or contaminated with carbon particles.
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