Shifts fine when it's cold but won't when it warms up

More than likely this is fluid related. So, we need to start with checking the level and condition.

Here is a guide that will help with this.

https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid

Once we know the condition of the fluid, we will be able to determine the best direction, but I suspect it will be low.

The reason is, when the fluid is low and it is cold, it is thicker and able to be pressurized better. Once it warms up and thins out it is not able to keep the pressure up and it basically goes to neutral.

The other likely cause of this is a pump issue or other internal leak that only comes in when it gets hot.

So that means if the fluid is ok, we are going to have to check the pressure of the unit to find out if the pump is building pressure. You simply put a pressure gauge in the line pressure port and then run the transmission in neutral and then in gear.

Please see the info below and let me know what you find with this, and we can go from there.

Thanks