Originally Posted by geeaj

Here is the procedure for the sohc found on the 89-90 s13

How to Read Malfunction Codes for Your SOHC 240SX
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On most car, only a jump wire or a screw driver is required to read the trouble code store on the engine computer. This the case for the 240 sx. The following intructions allow you to easily read and erase trouble code on your car electronic control unit (ECU)

The instructions come from the nissan 240sx 1989 12 valves engine service manual. There is a good chance that later models are similar.

In a few word, reading to code require accessing to the ECU, placing the unit in self diagnosis mode using a screw driver and looking at the flashing lamps on the unit to see what code is displayed.

1. Accessing to the ECU

The ECU is located to the right of the passenger footwell. You need to remove two fastener of the right doorstep in order to remove the kick panel. To remove the plastic fasteners, unscrew the center part and pull out the other part. Two metal screw and a fastener hold the kick panel. Some kind of pitch is also present.

Under the panel you now see the ECU with its connector. Unfortunaly, you have to access to the other side of the ECU. Unscrew the unit and turn it 180 degree on the vertical axis without unpluggin it.

You now see the front of the unit. An ajustment screw, a green inspection lamp and a red one are present. The screw is use to adjust the idle speed and to change the diagnostic mode.

MARK THE ORIGINAL POSITION OF THIS SCREW. UPON DIAGNOSTIC COMPLETION, RETURN THE SCREW TO THE PREVIOUS POSITION. OTHERWISE, ENGINE IDLE SPEED MAY CHANGE.

2. The ECU modes

The electronic control unit have five mode:

Mode I- Exhaust gas sensor monitor (Normal mode)
Mode II- Mixture ratio feedback control monitor
Mode III- Self-diagnostic
Mode IV- Switch on/off diagnostic system
Mode V- Real-time diagnostics system.

The unit automatically return to normal mode I when the ignition is switched off.

Only mode III will be studied here. Troubles code are stored on mode III. The stored codes will be lost if the car battery is disconnected or if after selecting mode III, mode IV is selected.

3. Changing to mode III.

- Turn the ignition 'ON'
- Turn the ECU adjusting screw fully clockwise.

The inspection lamps will flash onces, the unit will be in mode I. The inspection lamps will flash twice, the unit will be in mode II. The inspection lamps will flash 3 times, the unit will be in mode III. The unit will continually cycle through the five modes until the screw is turn back.

- Turn the screw back after the lamp has flashed 3 times.
- You are now in self diagnostic mode III.

4. Reading the trouble codes.

The codes are indicated by the number of both red and green lamps. First the red lamp flashes and then the green one. The red lamp correspond to units of ten and the green lamp to units of one. For example, when the red lamp flashes once and the green lamp flashs twice, this signifies the number "12".

The codes are:
11 Crank angle sensor circuit
12 Air flow meter circuit
13 Engine temperature sensor circuit
14 Vehicle speed sensor circuit
21 Ignition signal missing in primary coil
31 E.C.U. (E.C.C.S. control unit)
32* EGR function
33 Exhaust gas O2 sensor circuit
35* Exhaust gas temperature circuit
43 Throttle position sensor circuit
45* Injector leak
55 No malfunction

*California models only.

After the diagnostic, Turn the ignition 'off' to return to mode I.

To erase the memory, use mode IV. The codes are automatically erased from memory when the starter is operated fifty times after the trouble is last noticed.