jguo2002

I feel my 89 maxima stumble when idle or engine speed is below 2000. I checked the self-diagnose mode III on ECU according the repair manual, then result is code 55 what means no error. However, when I checked the mode I and II on ECU, the LED lamps did not blink at all. That means the Heated Oxygen Sensor circuit has wrong. I checked the circuit such as connector, input signal and ground continuity using multimeter, but there is no problem. It seems the Oxygen Sensor has problem, but why did not the ECU indicate the error code on Mode III?

And, when I disconnect the fuel pump fuse in order to check the ECU response, the ECU still indicated 55 on MODE III, Why? Is my ECU broken? Anyone knows the ECU please help me!

Pearl93VE

iTrader: (14)

First off, have you checked normal maintenance items such as spark plugs, wires, fuel filter, air filter, injectors, etc.?

It would be smart to start with the cheapest first (and the most obvious) if you haven't already. A bad ignition wire shouldn't light up a CEL.

Give us some more information to better diagnose your problem.

Oxygen sensors, however, do usually not result in stumbling, and the ECU in these car's rarely fail in the way you are describing.

jguo2002

Thanks for your response. The symptom is I can feel apparent shaking or vibrating while the shift is D or R and the engine speed is below 2000rpm.

Yes, I checked spark plugs, wires, fuel filter, air filter, but spark plugs, injectors not. I feel it is difficult to remove the injector, right?

Anyway, why did not the ECU response error code when I disconnect the heated oxygen sensor connector and fuel pump fuse?

Tquick

I'm not really sure why pulling the fuse didn't cause a problem, but still the computer is the least likely source of your problem, one of the hardest to diagnose, and one of the most expensive to fix so it should logically be the last place you look.

I would check all of the plugs by removing them from the engine, and looking at the business end. If one plug is significantly darker than the rest, it likely isn't firing. If the remainder of the ignition system is good, and you can verify spark at that plug wire, then it is time to consider that it might be an injector.

Pull the wire off the suspect cylinder with the engine running. If the shaking doesn't get any worse, then you know that cylinder isn't firing. If you verify that there is spark at the end of the wire, and the plug is good, then it is likely a failed injector.