How can I fix my catalylic conveter without paying almost thousand and the Dealership

Get a used transmission from auto salvage yard ! Several hundred dollars vs. several thousand dollars !

Get the dealership to enter the code they gave you! Or pay to get the correct code.....what make, model radio?

Have to pay someone with professional tools
to get the car ready for inspection

I wouldn't give my money to any dealers

You do realize no one ever has to go to a dealer,ever,
even with a new car,you own it,& as you said the dealers
are expensive
What about the couple hundred thousand repair shops that
do the same as a dealer?

try replacing battery first! if that don't work disconnect the truck battery for 10 minutes and try again!

It builds up back pressure on the motor and will not run if it is completely plugged. It can also ignite if it isn't fixed. Cars will sometimes still idle but have no power when you accelerate.

Unfortunatly,unless you have it written down in the owners manual that's where you have to go. I think a personal appearance might help your cause. But GM charges unless you go dirrctly to parts and ask for the manager.

if your check eng light is on have it checked out lot of times it will not be a major repair just a faulty solenoid witch is a lot less to replace than a overhaul good luck

Sun Visor:The only thing you can do,besides dealer,is go to a junk yard & find one to match what you have .It should be easy to put on.

The fault you are getting and with dealers recommending replacing the throttle body, it sounds like you have a defective engine wiring harness. I have seen quite a few of these. usually we will put a throttle on the car and then recommend an engine wiring harness if its not fixed. Not the best way to diagnose a problem but its almost impossible to find a fault with a multi-meter or an o-scope.It does not sound like the DME (engine computer) has failed but it could be a possibility. Also if you adjusted the throttle cable when changing the throttle. you will end up with the same faults. I recommend taking the car back to the dealer and being patient with them. this isn't the easiest thing to fix but most bmw dealership will take the time to make sure your vehicle is fixed correctly and will help you out any way possible