O/D Blinks but no CEL - How do I read Trans codes?

O/D light blinks after 15 mins but never any CEL. Code reader at Pep Boys reports no codes stored. How do I get at the transmission DTCs (I'll go to the tranny shop only as a last resort). Do I need to hook up to ODBII while driving when the O/D light starts blinking?? How is the tranny shop supposed to read these codes??

You will need to take it to a dealership or a transmission shop to get the code(s) read.
The O/D light flashing is like a CEL for the transmission....so a code has been stored.
This is not a OBDII code, but a manufacturer's specific code.