Saw the code but saw nothing wrong?
I had a vaguely similar issue at a dealer last year. They wouldn't install a GM warrantied part for me, as "they had never done it before on that type of car". The car was a 2007 GM product.
That shop was incompetent and the manager knew it, was the real story. I found another GM dealer's shop. You should too. They don't know what they are doing if they see an error code, and then tell the customer they can't find the fault. Finding the fault and satisfying you is the reason the shop
exists