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Code 51

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Well i did my first CEL code check today and i have to say it went really smooth. The fact that you can do your own troubleshooting on the vehicle is a definite bonus! Anyways after several attempts of checking code... clearing code to make sure i didnt goof the test up it would give me a 55 showing no malfunction and then after i started it and let it run the CEL would come back on and then i would check it and get a code 51. This confuses me because the pathy seems to run just fine! Starts right up idles smooth, drives down the road just fine. So my next Q's is what do i check now? Also was having speedo issues...after 30 mph the speedo goes crazy! So i took the advice of several on here and took the cluster apart and tightened screws and checked solder. Everything in the cluster seemed fine but speedo is still freakin. These few little probs are the only thing plaguing the pathy right now...im just really impressed with how she goes down the road. This is my first Nissan and so far im pretty impressed! So any help on this would be greatly appreciated and again this forum ROCKS!

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First thing I would suggest is downloading the Factory Service Manual to help in in any diagnosis.


Ok, code 51 is for the Injector circuit. Basicly what is going on is that the ECM is picking up a signal from the O2 sensor, that a particular "pulse" is either running rich or lean. Now, there can be several causes of this. First, the obvious. Thats a leaking injector. To find if you do have a leaking injector, I would pick up a fuel pressure test kit. This will "T" into the fuel line just after the fuel filter. Turn the key on, then off, and watch the fuel pressure. It should hold between 30-50psi for quite some time. If it drops sooner than that, you probably have a injector sticking open. How do you tell which injector? Well, first you can try unplugging them one at a time. This will only eliminate a electrical fault with the injector.


Next, I would pull the plugs and inspect them one at a time. A plug that has raw fuel on it, is probably getting wetter than it should. There may also be carbon buildup on the plug from not burning off during combustion. At this time, its also good to make sure the rest of your ignition system is operating properly(plugs, wires, cap & rotor). Any problems with those may cause misfire, possibly creating your code.


Now, there are other factors that may also cause this problem. Those could be the O2 sensor itself, Air Flow Meter, EGR system, Timing, Air leaks, the ECM and possibly something else(bad ground, ect).


Ask yourself a few questions quick so we know what condition your rig is in....


When was the last full tune up?

What other maintenence has been performed?

Anything else going on with the vehicle(ie:poor fuel mileage)?



Lets go from there and see where you get. Post up your results or other questions you might have! :aok:

Edited by 5523Pathfinder
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  • 6 years later...
  • 2 years later...

Id also like to know what is possible faults to check for. I got a misfire and i have occasional rough starts with a shaky idle for a couple seconds. the ECU gave me a code 51 for injector circuit. injectors were replaced, fuel filter replaced. it did help a little bit. im planning spark plugs next

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