Does the two letters need to match or the longer number on engine?

As far as I know, the old and the new computer engine have to be exactly the same, otherwise you’ll get a few or more issues, it depends on the manufacturer, the difference between the computers, the car model…

May I ask how did you come to the conclusion that you need a new computer, its pretty rare that you need a new one or get a used one to reprogram, although I do admit, it does happen on occasion. I would like to bend your ear for all the information please of what you or your mechanic tested please. Did you see something inside that was burnt, like an area on the circuit board??

Its very difficult to diagnose a car here on the internet especially since you may not have an oscilloscope to test the needed components, it may be possible to test certain items with a test light or a regular volt meter but many items create a signal to send back to the computer and many items would require a scope to be sure.

Reading codes is a good start but I don’t trust codes for a final analysis, I scope everything to be sure I am changing the correct parts and if I am going to call a computer, well, lets be extra sure, again, unless its burned up.

As for the correct used computer, lets first know why you made this call, will the used one also burn up and you can see that process starting already or do we need to do more diagnosis first…….. Little things on a vehicle will change the type of computer so you need to be super sure before wasting your money, most likely you will not be able to return it?? I would only trust the exact same numbers without a professional rebuilder, reprogrammer chiming in here to give their opinion.