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rosawoodsii 10-03-2019 07:28 AM

2002 Pathfinder 3.5L engine knocksensor problem
The engine light with the rear knock sensor has been on for about a year. I'm told repeatedly that it's not a problem and to ignore it. Now I've just had a ton of work done on the car, including new head gasket, tune-up, repair rear engine antifreeze leak, and more, took it home, and check engine light is now flashing, codes say knock sensor and misfiring on 3rd cyclinder. Back to the shop, replace coil, back home--and light is now flashing again, beeps from inside car, vibration, codes say 3rd cyclinder misfiring and rear knock sensor. Back to the shop.

The car is second hand, hood says 2000 3.3L engine, but it's actually a 2002 and mechanic says 3.5L engine, and he just can't imagine that the knock sensor has anything to do with the problems. Note: third mechanic to tell me this.

So, anyone have any ideas how to find the location of the knock sensors? And whether these problems (misfiring, vibration (even idling), CE light flashing, beeping) have anything to do with the knock sensors? The problems did not exist before I had the coolant leak taken care of and the engine all apart.

smj999smj 10-13-2019 01:51 AM

There is only one knock sensor and it is located on the top (in the "V") of the engine block, or, under the lower intake manifold. It will not cause an engine performance or drivability issue. If there is something that is causing and engine "knock," be it pre-ignition detonation, engine misfire or even a mechanical knock, i.e. a spun rod bearing, it will trigger the code and store it and the ECM will receive the signal from it and retard the ignition timing a few degrees in response, to help minimize damage to the engine. It, in itself, will usually not cause the CEL to turn on unless its circuit is shorted or open. For that reason, it is usually found with other codes. In those cases were there are more than one code, you should ignore the knock sensor code and address the other code(s). When the CEL is flashing, it indicates that there is a current problem that can cause damage to the catalytic converter. Most of the time it is a misfire of some sort and apparently, based on what you've told us, there is still an issue with cylinder #3. Sometime the wires break right at or in the harness connector for the coil pack. You may have a wire to that coil shorted or open somewhere. A bad fuel injector or injector circuit for cylinder #3 can also cause this. If it doesn't appear to be one of those, I would do a compression and cylinder leakdown test on #3 cylinder.

rosawoodsii 10-13-2019 07:58 AM

Thanks for the reply. That's a lot of good information.

I took the car back to the mechanic and they checked everything and found that a brand new spark plug
was misfiring on cylinder 3. For now, I'm watching it, but it appears that at least the misfiring problem is fixed.

The knock sensor code appears all the time, no matter what other codes are with it, though so far never by itself.

Usually it's with an exhaust sensor code.

smj999smj 10-13-2019 09:34 PM

When it comes to spark plugs, it's best to stick with the original equipment plug, which would be NGK # 6240. Aftermarket plugs, especially Bosch platinum and Splitfire plugs, can cause performance issues with the engine. A bad oxygen sensor often causes an oxygen sensor code to trigger and if it's an upstream sensor, it can cause drivability issues by causing the engine to run too lean or too rich. Too lean can cause detonation which triggers the knock sensor. There are other things, however, than can cause an oxygen sensor and not be the sensor, itself. If an engine is running really lean, the oxygen sensor will respond with a consistent reading on the lean side and the ECM may interpret that as the oxygen sensor have an issue, triggering the code.

rosawoodsii 10-14-2019 07:38 AM

Thanks. More good info. I'm not sure what kind of spark plugs the mechanic put it. He said they were expensive, not low end, so he was very surprised to find a bad one in the lot.

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