03 LT EXT OBDII Not working

I have a 03 that my check engine light came on when the engine started running alittle rough, like idling @ 900 its spuddering, when speeding up, the light flashes, then let off and coast, the light goes solid, so I hooked up my scan reader and.....nothing, wouldnt even come on.
So I hooked it up to my wifes car and it came on just fine and told me no codes present, so tried agin on Trailblazer and again....nothing.
Any ideas???
I need the codes so I can figure out whats wrong with it.

Well, it's obvious a P0300 is set here......when a misfire is so bad, that it could harm the cat, the CE light will flash.....when you ease up on the gas, and the misfire lessens , the light goes out....

What kind of code reader do you have? Some can't read a system if it is a CAN run system.....Did it ever work on this vehicle before?

What vehicle does your wife drive?

Places like autoZone will read the code for free....

But I think your main problem is a bad misfire,,,,,,could be secondary ignition or an injector problem....

I hate to comment on an older post but if anyone else is having this problem too my wife's 06 trailblazer had the same problem hooking up a code reader. There was a blown fuse for the cigarette lighter outlet which apparently powers the OBDII as well. It's a 20 amp fuse if I remember correctly and it's in the fuse panel under the hood. Popped a new one in because the one that was in there was bad and presto the code reader worked just fine.