The ECU is located behind the front passenger kick panel, located in the passenger footwell.

Here are the instructions for reading the LED light located on the ECU. The number of blinks correspond to the trouble codes below.

If a malfunction occurs within the ECU/fuel injection system, a diagnostic trouble code may be stored in the ECU and the ‘‘Check Engine’’ lamp will blink. There is also a lamp built into the ECU which will concurrently blink. To read Code output, proceed as follows:

1. Turn ignition On, but do not start engine.
2. Using a small screwdriver, turn diagnostic mode selector switch, located on side of ECU, fully clockwise.
3. Wait two seconds, then rotate switch to full counterclockwise position.
4. A malfunction code can be read as the long (0.6 second) blink, indicating the first digit of a two digit number and the short (0.3 second) blink, indicating the second digit.

Ensure that the diagnostic switch is to the full counterclockwise position whenever the vehicle is in use.

Once you have the numeric code, here is what they mean:

11 Crank Angle Sensor
12 Mass Air Flow Sensor
13 ECT Sensor
14 Vehicle Speed Sensor
16 TCS Signal
21 Ignition Signal Circuit
25 Idle Air Control Valve
22 Fuel Pump Control
31 ECM
32 EGR Function
33 Heated Oxygen Sensor
34 Knock Sensor
35 EGR Temperature Sensor (RH)
41 Intake Air Temperature Sensor
42 Fuel Temp Sensor
43 Throttle Position Sensor
45 Injector Leak
46 Secondary Throttle Sensor
51 Injector Analysis
53 EGR Temperature Sensor (LH)
54 Signal Circuit From A/T Control Unit To ECM
65-71 Single Cylinder Or Multiple Cylinder Misfire
72 Three Way catalyst Function
76 Fuel Injection System Function
77 Rear Heated Oxygen Sensor
82 Crankshaft Position Sensor
84 A/T Diagnosis Communication Line
91 Front Heated Oxygen Sensor Heater
103 Park/Neutral Position Switch
105 EGR & Canister Control Solenoid

http://forums.nicoclub.com/zerothread?id=270520

Thanks littlewillie!