I have a 2004 Trailblazer with 45k mi., the service engine soon light came on. I took it to Autozone and Advance auto to have the code read. When they hooked up the code reader, it would not read a code, like it was getting no power. Has anyone ever heard of this happening before?

 

check your fuses...specifically the one for the cigarette lighter

 

like he said.
the OBD port gets its power from the same circuit as the Cig lighter.
check that fuse to make sure its not blown.

 

Just wanted to say thanks, that was the problem..