Check Engine Light ON, Retrieving Codes?

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OK, i read how to get the codes from an ECU but, i am unclear on things. It is a '95 Pathfinder XE with the V6 Auto. Some threads say to use the "Check Engine " light to retrieve the code and others say to use the Green & Red lights on the ECU to retrieve the codes. Was there a mid year ECU change for this vehicle that year? My computer has a dial with LOW, HIGH, Mode Change, Idle Speed Adj., and Diagnosis written around it and the red and green lights. What does all this mean and how do you get the codes from this ECU?? Ant