Originally, Nissans carried two-mode self-diagnostics, but by '87 all models were endowed with a five-mode system. That's what we'll explore here, specifically the '91 federal version found on the popular Maxima.
To reiterate what we've said in every electronic engine control article we've done over the last couple of decades (and that's a lot), don't assume that the ECCS is the source of the complaint just because it's present and complicated. Look for traditional, basic troubles first: fouled plugs and faulty ignition components, weak or uneven compression, vacuum leaks (as with other computerized systems, unmetered intake air will reduce idle speed), restricted intake, a cam drive that's out of synch, improper fuel supply pressure and volume, low voltage or poor connections in the electrical system, and even such things as transmission and A/C operation. Also, do a thorough visual exam of the computer system's wiring and connectors, including those extremely important grounds. You may want to check oxygen sensor activity with a DMM or dedicated tester, too.
But if all that is okay, and/or the Check Engine light is on (Nissans didn't get this warning device until '88 for California and '89 federally), the next logical step is to engage the self-diagnostics. Unlike most other carmakers, Nissan has you go right to the ECU for this. Remove it from the passenger's side kick panel, then find the mode selector screw and the port in the housing through which a red and a green LED are visible.
Switch on the ignition, turn the selector screw clockwise all the way, and watch the LEDs. They'll flash once, then pause, twice then pause, and on up to five times to indicate the five diagnostic modes. When you see the number of the mode you want, turn the screw counter-clockwise.
*Modes & codes
Mode 1 is called the "Exhaust Gas Sensor Monitor" because it informs you about oxygen sensor activity. With the engine running and fully warmed up, the green LED should blink, going on when the sensor sends a lean signal and off when it sends a rich signal. You should see 5-10 flashes every 10 seconds. If the LED is on more than it's off, there's a lean condition, and vice versa. Sluggish blinking should make you suspect a fouled sensor.
Mode 2, the Mixture Ratio Feedback Control Monitor, lets you know whether or not the air/fuel mixture is being controlled within the proper range. If the red and green LED's flash pretty much simultaneously with the engine running, mix control is okay. If the red LED is off more often than the green one, richness is indicated. If the red is on more than its mate, think lean.
Mode 3 has the name "Self-Diagnostic," and it's more what you're used to on other vehicles because it yields fault codes, both hard and intermittent, which are communicated to you by means of the flashing of both LED's. The red one gives the first digit, and the green one the second digit. For instance, if the red flashes twice, then the green flashes once, you've got a Code 21.
Get the engine up to normal temp, preferably by driving the car for 10 minutes, then pull the codes. In the case of a no-start, crank the engine for at least two seconds. If you see 55, all is well (in older models, 44 meant the same thing). Be sure to write down any other codes because they'll be erased when you go on to Mode 4, which means you will have lost any help on intermittents. Normally, codes are retained in memory for fifty starts.
Mode 4, called the "Switches On/Off Diagnostic Mode," checks the function of the switches that serve as inputs to the ECU, specifically those for ignition key start position, idle, and vehicle speed. For the first two, the red LED will go on or off when switch status is changed. In other words, with the ignition on, the red light should illuminate both when you step on the gas pedal and when you turn the key to start. If not, check the appropriate circuit. The vehicle speed sensor lights the green LED when you exceed 12 mph (get the drive wheels off the floor to do this in the shop, or have a helper drive while you keep your eyes on the ECU).
Mode 5 ("Real-Time") gives instantaneous trouble codes to let you know what's going on right now in four monitored circuits. Codes are flashed out just once when a malfunction is detected, then they're immediately forgotten. And the way you read them is different -- you observe the flashing of one LED or the other, the red one reporting on the crank angle sensor and the fuel pump circuits, and the green on the air flow meter and ignition signal circuits.
If the red LED pulses out a series of long flashes separated by equally long pauses, a malfunction in the crank angle sensor or its circuit's indicated. Groups of three short flashes of the red LED point to the fuel pump or its circuit. With the green LED, two medium flashes followed by a pause, then two again, and so on, should cause you to suspect the air flow meter and its related wiring. Groups of four flashes mean there's a problem with the ignition signal.
This is an extremely sensitive and very useful mode. You can wiggle wires and connectors, rap on components, and have somebody else drive the car while you watch for those fleeting indications of trouble. You'll know everything's okay if you see no flashing in five minutes of revving and idling.
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