The "check engine" light comes on and a message displays "power diminished"

What is your question?

This happens intermittently, when engine is started, and usually remains a problem through entire trip. Problem continues for a few more trips, then disappears until the next time. With diminished power, acceleration is very slow, but car remains drivable.

What makes this problem better or worse?

Happens in dry, wet, cold, warm weather. No specific cause.

How long has your 2016 Buick Regal had this problem?

Since I bought the car, last year. Dealer tried to identify cause without success.