Obd codes

why won’t my 2000 bravada show certain codes when plugged in to a diagnostic machine?

You need to clarify the question. You will only get codes that the computer has set b/c it has detected a system or system part out of spec. It won’t show codes if it doesn’t see a problem.

Or do you mean that your engine light is on, but no codes are showing when it is hooked up?

Are You Looking For Body Control Module Or ABS Codes? Your Diagnostic Machine May Not Be Equipped For Them.
CSA

the engine light comes on,and no codes show up

Check with a second code reader and report back.

Is it reading ‘No Codes’, or is it failing to connect? There is a big difference. I’ve had problems sometimes not able to connect that was solved by finding a blown fuse.