96-on requires a scan tool, 04-on requires aCAN scan tool.
Steve, at least for the '99 - '04 Lightings, there is a built-in check that is fairly accurate. I say "fairly" because there have been a few times when my diagnostic scanner has picked up codes, and the OEM "cheater" has missed them.
OEM method for checking codes:
After you get in your truck, before you turn the key at all, press and hold the trip reset button.
Turn the key to the ON position, and the lights on the odometer read-out change. Let go of the button. You are now in a scroll-through menu display. Each successive pressing of the reset button once will scroll you through the menu to the next category.
If you press and hold it too long, it will clear out and go back to the mileage.
<O</OLook for the menu that says DTC.
If you have a code, it will say DTC2145 or something similar. Or it may display DTC NONE, for no codes.
Another cool feature of this readout function is a digital RPM gauge.
Once you turn your truck off, the readout resets to the default odometer reading.