CEL on Datalogger setup

I was trying to pull the CEL on my datalogger, and it says the CEL is on, but it wouldn't give me any codes. There's options on it thats like Multiplex, Disbales, ISO, and some other things. When it's set to the Mulitplex->ISO, it picks up the sensors, but it still wont get the CELs. What am I doing wrong here? Thanks.