The LED box allows you to draw codes from the diagnostic connector near the battery next to the heater control valve. These codes are for subsystems only such as SRS, ABS, and CCU. The engine codes are drawn from the EZL ignition module mounted on the left inner fender well. The way that codes are drawn from there is by measuring duty cycle from pin 3 of that connector to ground.

I'm sure that the dealers had/have a fancy analyzer to connect to the EZL diag connector, but there are only 10 or less codes that can be drawn from there. Each code is indicated by a solid duty cycle that is a multiple of ten. If the duty cycle is not stable then there is no code present.

Have a great day,