BamaMax

OK. A couple of weeks ago, my SES light comes on. I check it out and reset the light. (it was 0302, EGR). So, I don't have much time to check into any further. A couple of days later, it comes on again. When I check it, it's the same code, so I reset the light. A couple of days later, it comes on again. I don't bother to check or reset because I know what it is and I was going to check it out over the weekend. So I drive a day or two with the light on, but, when I started to leave work the other day, I got in and started the car and the light was off. ? The bulb is not blown because it still lights up briefly when I turn the key on. Any ideas??

Thanks,

Daniel B. Martin

The Engine Control Module make checks on vehicle operation on a continuous basis. Some tests might be done only once per trip (example: fuel system integrity). Other tests are done on an on-going basis (example: Oxygen Sensor signal range check).

The ECM logic can decide that a previously recognized problem no longer exists and it will extinguish the Malfunction Indicator Lamp without a manual reset.

If there really is an EGR problem with your engine, that annoying Service Engine Soon light will come back sooner or later. Probably sooner.

BamaMax

Thanks DBM!!! I knew you would shed some light on the situation.

Daniel B. Martin

I did some research in the '99 Maxima factory service manual and came up with more specific information on this subject. I will post it as a seperate message on this thread, and also send it to Bill99GXE for possible inclusion in his ECU Diagnostic Trouble Codes Sticky.

Daniel B. Martin

The Malfunction Indicator Light (a/k/a Check Engine Light, Service Engine Soon light) is controlled by the Engine Control Module (the computer). The ECM routinely makes a variety of checks on vehicle operation. Some checks are made only once per trip (example: fuel system integrity test) while others are continuous (example: Oxygen Sensor signal range check). In this context a "trip" means driving the car long enough for the engine management system to go into closed loop mode.

When the ECM detects a problem it turns on the MIL and stores one or more Diagnostic Trouble Codes to indicate the nature of the problem. (Note an exception: a Knock Sensor problem will set DTC 0304 but not turn on the MIL).

Even with the MIL on, the ECM continues to make the routine checks. If the car completes three successive trips in which the problem did not appear, the ECM turns off the MIL on the premise that the defect was remedied or went away without intervention. However, the DTC which points to the problem persists is not immediately erased. The DTC is preserved for "N" problem-free trips, and the value of N depends on the nature of the problem. If the problem involved the fuel injection system or an engine misfire, N = 80; for any other kind of problem N = 40.

Understanding the MIL "turn on - turn off" logic teaches two things ...
- Don't be alarmed if the MIL turns off without a manual reset. That is normal.
- Don't cancel an upcoming dealer appointment thinking that the symptom is gone. The DTC will persist for a while.