The Electronic Control Unit (ECU) in all vehicles will store and report fault codes. If a malfunction is detected only one time, the CHECK ENGINE light may not stay lit. However the fault code will remain in memory for the next 50 ignition cycles when the starter is operated. If the fault occurs again, the counter will reset to 0 and the code will be in memory for 50 ignition cycles from that point. The fault code can be erased manually and is erased any time the vehicle battery is disconnected. If the ignition switch is turned OFF when in diagnostic mode, the ECU automatically defaults to Mode 1.
There are two diagnostic programs used by Nissan, a four mode and a five mode. All vehicles covered here use the five mode program. Modes 1 and 2 monitor mixture ratio feedback control. Mode 3 reports trouble codes and is the mode used for clearing the stored codes. Mode 4 monitors various switches in the engine management system. Mode 5 displays faults occurring at the moment of operation and can be used while driving the vehicle.
As useful as these diagnostic tests are, the first step in repair or service to engine management systems is still to gain and evaluate as much information as possible regarding intermittent problems and to repair the obvious. Before checking fault codes, it is absolutely essential to check for any obvious mechanical faults or failures. Remember, a trouble code only indicates which sensor or circuit is effected by the problem. Simple mechanical faults such as a vacuum leak or poor electrical connection can cause a fault code.
The Electronic Control Unit (ECU) for all models is located under the front passenger seat. If equipped, the ECCS check connector is located in the instrument panel to the rear of the hood opener assembly. Always refer to the diagnostic charts for additional service information.
ENTERING SELF-DIAGNOSTIC MODE
- Locate the ECU and remove it from its bracket to gain access to the mode select switch. Do not disconnect the wiring.
- With the ignition switch ON , turn the diagnostic mode selector switch (a small slotted screwdriver is usually necessary to reach the 3 position knob) to the ECU diagnostic position (fully clockwise), hold it in this position and wait for the inspection lamp to flash.
- Count the number of flashes, and after the inspection lamp has flashed the number of the required mode, immediately turn the diagnostic mode selector fully counterclockwise.
- When the ignition switch is turned OFF , the ECU automatically returns to Mode 1. When the battery is disconnected, or when manually leaving Mode 3 or 4, the stored fault codes will be automatically cleared.
- With the ignition ON , engine not running, both the green and red LEDs will be lit.
- With the engine running, the green LED will stay on or off until the engine and oxygen sensor are warm enough to go into closed loop operation. At that point the green LED will blink on and off.
- The red LED will be off unless the vehicle is a California model and has stored fault codes.
- With the engine running in closed loop operation, the green LED will blink on and off.
- If the red LED blinks with the green one, the air/fuel ratio is cycling correctly.
- If the red LED stays off, the mixture is more than 5 percent rich.
- If the red LED stays on, the mixture is more than 5 percent lean.
- If the red LED stays on or off during closed loop operation, complete the diagnostics before beginning repairs.
All fault codes are two digits and both LEDs are used to report the fault codes. The red LED is the first digit, the green is the second digit. If the red LED flashes 2 times, then the green flashes 4 times, this indicates code No. 24.
- With the engine running in closed loop operation, operate the Mode select switch to enter Mode 3. The red LED will flash first, then the green one.
- Count the flashes and write them down. Allow the cycle to repeat a number of times to make sure they have been correctly read.
- If code 55 is the only code displayed, no faults are in memory.
- When the ECU is switched to any other mode from Mode 3, the stored fault codes are automatically erased, including intermittent faults. This can be avoided by turning the ignition switch OFF . The ECU will automatically leave diagnostic mode without loosing the stored fault codes.
These switches are monitored by the ECU: the throttle and idle switch, starter switch and the reed switch for the speed sensor. In Mode 4, each switch that is turned on or off will cause an LED to flash. If no LED flashes when the switch is operated, the switch circuit, the ECU, or switch itself is faulty. When the ECU is moved out of Mode 4, the fault codes are automatically erased. To leave Mode 4 without loosing the codes, turn the ignition OFF , then the ECU will default to Mode 1.
- With the ECU in Mode 4, turn the ignition switch to START . The red LED should flash when the starter switch is turned on and again when the starter switch is released.
- With the ignition ON , with or without the engine running, the red LED should flash whenever the throttle switch is turned on or off. The throttle switch signals the ECU that the throttle is in the idle position.
- When the vehicle is moving at more than 12 mph (20 km/h), the green LED should flash or stay on.
In Mode 5, the ECU will report one of 3 fault codes while the engine is running: malfunction of the crankangle sensor, the ignition signal, or the air flow meter. These codes are not stored, but can be reported while the engine is running or while vehicle is being driven, if desired. Should the ECU detect a malfunction, an LED will flash on and off at different rates for each sensor. The codes are reported differently from Mode 3. If no fault is detected, no LEDs will flash.
- With the engine running, put the ECU into Mode 5.
- Watch the LEDs for more than 5 minutes while the engine is operated at different rpm. If any LEDs flash, write down the number of flashes and note the time between groups of flashes.
- Compare the notes to the diagnostic codes and related diagnostic charts, to find which of the sensors (or circuits) is faulty.
This diagnosis procedure is a very effective measure to diagnose whether the above systems cause the malfunction or not, during a road test.