I don't know about GM products but on some of the Fords you can just jumper two of the pins on the diagnostic connector with a paper clip. Your trouble codes will flash up on the check engine light. ( Paper clips don't cost much. )
The main problem is that it's too easy to end up chasing your tail with just a little bit of knowledge.
I've known people that have spent hundreds on blindly replacing sensors and electrically operated valves only to have the problem get worse.
You really need a good ohm meter and a vacuum gauge to test the different parts & pieces before replacing them. Sometimes it's in the wiring that runs from the sensor to the ECU. Sometimes the aftermarket parts are inferior to the OEM parts and they'll give you a bad reading.
Lotta ways to run into problems if you're trying to do the DIY thing.
ETA: As a guy that's just a shade tree mechanic i found this link to be super helpful. It breaks it down to the basics and explains fuel injection in a way that i could understand.
Fuel Injection Technical Library
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