After driving for a few minutes, the transmission will not shift into fourth gear?

Hi,

It could be the result of a few things.

Here is what I suggest. We need to scan the can-bus system. CAN stands for controller area network. Basically, the different computers/modules are tied together via a few wires. This type of scan will retrieve codes regardless of the module storing them.

Here is a link showing how this is done:

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Let me know if you are able to have that done. If you can, let me know what codes are retrieved. If you can't have it done, let me know that as well.

Take care,

Joe