Hello, The key symptoms are heat and the effect of the car in gear. Check the transmission fluid to see if it is burnt smelling or very dark. The transmission may be binding like trying to be in two gears at once.But I think there may be an electrical problem. To find this may take some time and a great flashlight. One scenario is this; The movement of the engine as it is engaged in gear causes a wire to either ground out on a piece of metal, or it looses connection due to loose wire terminal or damaged /cut wire. To find this out you need to watch the movement of the motor and inspect for any interference the wiring may have with other parts of the car. You can also carefully start moving the wire harnesses about where interference may be, like around tight corners or where it contacts the motor. Check the wires going into connectors and the connectors themselves. Also check for wiring that may be touching the exhaust, drive axles , belts or pulleys. Move the wires easy at first ,and have someone hold the car in gear for you ,with the brakes on of course. Revving above idle may show you where the wires may be stretching or being crushed. Be very careful not to stand if front of the car as it may lurch forward unexpectedly.
A bad fuel pump acts differently. and a catalytic converter is unlikely because direction of movement should have no effect on a broken one.
You most likely will find that a wire is rubbed thru and shorting against metal or another wire.
I have had trans bind up and have been able to free them up by revving the motor and putting them in gear. I was not worried about breaking them because they were already damaged. The jarring did free up a couple of them , and one never had another problem like that again. I hope this may help. Please ask if there is any more need.
The U 1301 code relates to high voltage of the communication lines, This means that there was /is a short to voltage on the serial communication line. With a car that's been in a accident and put back together, there could be dozens of things including connecting broken wires incorrectly . Electronic modules shorting out internally. I recommend having the Saturn dealer scan the car with their tools. Its best to write out all the info you have on the car, like its been rebuilt, and the sensors changed. If you still have the old sensors leave them in the car and mention this in your note to the Tech.I work on lots of rebuilds and I don't think any two problems are ever alike. Some times experienced Tech need to look at these types of problems. It will save your more in the long run. Its hard or impossible to returned a fuel pump, sensors, or computers.
I don't have that wiring diagram availible to walk you through. The class 2 refers to the communications lines that are effected. You will need a factory wiring diagram and diagnostic manual to trace these wires to all the modules they interconnect to. These wire are used to allow all the computer modules to share information . Sort of like telephone wires. So they connect to the engine, body, anti-lock, transmission modules ect...So without a service manual it would be very hard to track these wires. I was hoping that giving you the basics may allow you to find an area that may be damaged. Don't overlook areas where battery acid may have leaked onto the harnesses. This problem you have requires special scan tools and information that I cant provide. So i must opt out. Scott.