What causes a PO826c code and is it hard and expensive to fix?

What is your question?

Model LS 3.5 engine. I got this code along with PO449b & PO455b. I had a very bad and loose gas cap and have just recently replace it with an OEM cap from the dealer. Once I got the new cap the shop reset my engine light and said to see if it stayed off but as soon as I filled up and restarted it came back on. Why would a low range up/down shift switch affect my inspection status in the first place? I almost never take it out of drive.