My pontiac Gran Prix is stuck in third gear, why?

https://youtu.be/c3rHuIKEUiw

Well, since the transmission in your vehicle is computer-controlled, I think the FIRST step in diagnosing the problem should be to ask the Powertrain Control Module what is going on.

Below is the diagram you requested. Best of luck and thank you for using Fixya!
Greg

2004 Pontiac Gran Prix 3.8L

If leaking coolant under dash on passenger side floor broad it's your heater core. Call's for 2.8 labor flat time to remove heater core, in some case's you'll have to recover, evacuate and recharge your A/C system to remove your heater core.Sit under dash on passenger side.

The upscale model is the only that i know of that has one. What model do you have?

go to the dealer with the vin and proof of purchase and they should be able to cut a key with this information you will also have to have proof of ownership.

get the codes on trany OBD2. what does the over drive button is saying on the dash Vitally

i do believe you have to take the passengers side dash apart behind the glove compartment. go to autozone and get a manual or go to the library and check the chiltons manual. this is not an easy job. how bad is the leak? if its not bad these heater cores are made of all aluminum. i would buy a bottle of that radiator additive and see if that takes care of the problem first. it only cost about 5 bucks a bottle and it may permanantly fix your problem.

there shoud be a little torx scrw holding it on remove it its either a t10 or t15 torx driver