Self Diagnosis

The ECU on the 89-90 SOHC 240sx is found in the passenger front kick panel. Remove the panel, and 2 10mm bolts and pull the ECU out. If you look on the back, you can find the diagnostic mode selector as well as the inspection lamps.

The following is verbatim from the Nissan 240sx 1990 Factory Service Manual.

Self-diagnosis -- Description

The self-diagnosis is useful to diagnose malfunctions in major sensors and actuators of the E.C.C.S. system. There are 5 major modes in the self-diagnosis system.
1. Mode I (Exhaust gas sensor monitor)

During closed-loop operation:
The green inspection lamp turns ON when a lean condition is detected and goes OFF under rich condition.
During open-loop operation condition:
The green inspection lamp remains OFF or ON.

2. Mode II (Mixture ratio feedback control monitor)
The green inspection lamp function is the same as Mode I.

During closed-loop operation:
The red inspection lamp turns ON and OFF simultaneously with the green inspection lamp when the mixture ratio is controlled within the specified value.
During open-loop operation:
The red inspection lamp remains ON or OFF.

3. Mode III (Self-diagnostic system)
This mode is the same as the former self-diagnosis in self diagnosis mode.

4. Mode IV (Switches ON/OFF diagnostic system)
During this mode, the inspection lamps monitor the switch ON-OFF condition.

Idle switch
Starter switch
Vehicle speed sensor

5. Mode V (Real-time diagnostic system)
The moment the malfuction is detected, the display will be presented immediately. That is, the condition at which the malfunction occurs can be found by observing the inspection lamps during driving test.

HOW TO SWITCH THE DIAGNOSTIC MODES
1. Turn ignition switch "ON". 2. Turn diagnostic mode selector to E.C.U. (fully clockwise) and wait for inspection lamps to flash. 3. Count the number of flashes, and after the inspection lamps have flashed the number of the required mode, immediately turn diagnostic mode selector fully counterclockwise.

When the ignition switch is turned off during diagnosis in any mode and then turned on again (after power to the E.C.U. has dropped completely), the diagnosis will automatically return to ModeI.
The stored memory will be lost if:
1. Battery terminal is disconnected.
2. After selecting Mode III, Mode IV is selected.
However, if the diagnostic mode selector is kept turned fully clockwise, it will continue to change in the order of Mode I -> II -> III -> IV -> V -> I ... etc., and in this state the stored memory will not be erased.

This unit serves as an idle rpm feedback control. When the diagnostic mode selector is turned within the "diagnostic mode OFF" range, a target engine speed can be selected. Mark the original position of the selector before conducting self-diagnosis. Upon completion of self-diagnosis, return the selector to the previous position. Otherwise, engine speed may change before and after conducting self diagnosis.

Mode III Code Table

Code No	Detected Items	California	Non-California
11	Crank angle sensor circuit	X	X
12	Air flow meter circuit	X	X
13	Engine temperature sensor circuit	X	X
14	Vehicle speed sensor circuit	X	X
21	Ignition signal missing in primary coil	X	X
31	E.C.U. (E.C.C.S. control unit)	X	X
32	E.G.R. function	X	-
33	Exhaust gas sensor circuit	X	X
35	Exhaust gas temperature sensor circuit	X	-
43	Throttle sensor circuit	X	X
45	Injector leak	X	-
55	No malfunction in the above circuit	X	X

I hope this helps. If you have any questions, drop me an email here: brad at sph33r.net

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