DTC not communicating
 

I have a 2003 Tahoe, 4.8L, & 196k miles. Just had a messege scroll that red "reduced engine power" Car still ran but slow to excelerate. Got home plugged in OBD2 reader and reader did not power up. I connected reader to my other vehicles and read worked fine. Disconnected / reconnected battery and tried reader again and nothing. Before this car ran fine.

Welcome to the forum.:) When you disconnected the battery, whatever code got cleared. Is it still in limp home mode? The reader not powering up or not connecting? Check the OBD data link connector for battery voltage on pin 16 (rightmost pin, bottom row as you view the connector under the dash) and ground on pin 5 (5th pin from the left, top row). If battery voltage absent, check the 15A CIG LTR fuse.

You were right. A blown fuse. Thanks.

You're quite welcome.:) Hopefully you can read out some useful trouble codes next time it acts up.

Bingo
 

this was my issue. solved