For a 91 R/T, simply connect wire from ground through a 470 ohm resistor to the cathode (-) of an LED and the anode (+) of the LED to pin 1 ( upper left pin on diagnostic connector ). LED will continually flash error code anytime the key is on. My code 53 was 5 long flashes, pause, and 3 short flashes. This code repeated continually until reset by removing battery cable for a few seconds erasing memory. With no error, light flashes porportionally on/off like the curser on your computer. As soon as you get a check engine light, the light immeadiatly flashes the error code and repeats. Cool !!!