The ECU is in the passenger footwell, behind the plastic kickplate to the left of your feet. You'll have to remove the computer from the mounts in the footwell, as the led/leds you'll need to see are pointing towards the front of the car. Switch on the ignition but don't start engine, the led on the ECU will begin glowing turn the selector screw clockwise all the way.

There's three types of factory ecu in Skylines, One type has two LEDs and five modes, the other type has one LED and two modes and the last type has no LEDs, the output is done with the engine check light in the instrument cluster.

For two led ecus the led will flash once, pause, flash twice, pause... all the way up to five this is the diagnostic mode Mode 1 = one flash, mode 5 = five flashes. When you reach the right mode, turn the screw back counter-clockwise. Mode 3 is what most people use.

For one led ecus wait a couple of seconds, and turn the screw back counter-clockwise, the ECU is now in mode 2 the engine check light on the dash will mirror the LED on the computer.

The led will flash out a code or series of codes if there is more than one fault More recent ECU's have two LED's red is first digit, green is second digit, eg red 2x flash, green 1x flash = code 21. The ecus on most R32's have one LED, long flash = first digit, short flash = second digit, eg long, long, short = code 21.

Codes:
11 Crankshaft position sensor
12 MAF sensor circuit
13 Coolent temperature circuit
14 Vehicle speed sensor circuit
21 Ignition circuit
31 ECU
34 Knock sensor
43 Throttle position circuit
45 Injector leak
51 Ignition circuit
54 Auto signal to ECU
55 All OK
Turn the ignition off to put the ECU back to normal.

There is also a O2 sensor test in real time available - do the above to get the ECU into mode 2 (single LED ECU) or mode 1 (2 LED ECU's) Start the engine. Warm the engine, and then run under no load at 2000 rpm for the test. Single LEDs: on = lean, off = rich Dual LED's: green LED on = lean, green LED off = rich