I've read certain vehicles may begin flashing their check engine light if one bridges the diagnostic plug correctly with some wire before starting the vehicle. How does one do this for a Pontiac G6? I've attached a diagram.
As this article points out, the ability to blink codes was available with older OBD-I vehicles. Once the OBD-II standard hit, the ability to "blink" out codes was lost. This changeover to ODB-II happened in 1996 for vehicles sold in the US.
I know the same is true for Volkswagen cars. Early models had the ability to blink out the codes when something was jumped, but modern cars needed a special reader.
This bit is conjecture on my part, but I think the reason is that OBD-II is much more complex so the simple blinking was just abandoned.