Having trouble with a 2005 cobalt. Sometimes car just goes crazy tranmission shifting weird codes are U2100,po700 and po742 think it is in the can bus but dont know were to start?

try a fluid change

A sensor is going bad in your shifter assembly. When you shift the car out of Park the doors automatically lock. The sensor known problem

P0700
P0700 OBD II Trouble Code Transmission Control System Malfunction...
p0732
P0732 OBD II Trouble Code Gear 2 Incorrect Ratio YourMechanic Advice
p0733
P0733 OBD II Trouble Code Gear 3 Incorrect Ratio YourMechanic Advice

You need to replace the "B" shift solenoid inside the transmission.

tranny codes if its not shifting get a good used tranny

Means that the scan tool the shop uses can not communicate with your computer. This tells me that there is a broken or shorted wire somewhere between the connector and computer in your car, or the computer in your car is bad.

Did you actually erase the codes after the first time?
Sounds like the speed sensor lost contact first, then the transmission went into limp home mode? So everything's fine now, contact was restored with the speed sensor. BCM is the body control module, kind of a nutty professor's idea to centralize a lot of body electrical wiring into a single control module, seems to cause more problems than it alleviates. Wouldn't be surprised if the speed sensor and the fuel level, like the rest of the dash cxluster is not all routed through the BCM. Well, it works now. When the problem re-occurs, you'll maybe be able to snuff it out. Bood luck.

I had a similar problem with my 2005 Cobalt LT. Mine was as simple as checking the engine oil level. When the engine oil gets too low it will make the car do all kinds of weird stuff.

THERE IS ONE COMMON FACTOR ABOUT COMPUTERIZED CARS "IF YOU HAVE A LOOSE OR FAULTY GROUND WIRE "THE DIAGNOSTICS MACHINE WILL READ FAULTY THUS MAKING IT A TRIAL AND ERROR PROBLEM AT THE DEALER
WHAT I RECOMMEND IS CHECK OR HAVE YOUR GROUND WIRES CHECKED THERE SHOULD BE ONE FROM FIREWALL TO BLOCK,1` FROM BLOCK TO CHASSIES,1 FROM BATTERY TO BLOCK ,1 FROM BATTERY TO CORE SUPPORT AND PROBABLY MORE CHECK REPAIR MANUEL FOR ADDITIONAL GROUND WIRES