98 Chevy 350 error codes ????????

I drive a Freightliner for a living and know that on them you can see what and where anything that is wrong is w/ the engine by running an engine diagnostic (counting how many times the check engine light blinks and then consulting a list). Which brings me to my question.

I have a service engine light on in my pickup. A '98 Chevy half-ton 4X4 w/ a 350. It acts like it is loading up. To be sure though I was wondering if there is any way to run a diagnostic on the engine. On the Cat's you you turn the cruise off and hold the accel. switch for about 5 sec.s. I've been told you can short out a couple terminals on the service port, which I have tried (to no avail) am I doing it wrong or is there another way to do it?

Any help would be greatly appreciated!