Check engine light comes on. And then when you try to pass a car it will not down shift into passing gear. Auto trans. The light turns off then it shifts normal.
This sounds about right. If the PCM detects an issue with the transmission the check engine light will illuminate and may keep it from shifting. This can happen when dirt inside the transmission contaminates the fluid. Thus hindering the proper function of the internal components of the transmission.
The good news is the PCM records the trouble code and stores it. This means you can have a technician hook up a scan tool and determine the code. The code will provide insight into what exact component is having an issue.