After i recently posted something about having the check engine light come on, and i did the codes and it gave me rear 02 sensor and knock sensor, i thought to myself about what the hek they really were!!!
I dont even know what they are, i know it sounds pathetic, but this is my first car, a 1995 Pearl White Metallic Nissan Maxima SE....and proud of it!!!!

I just dont get it though..can someone tell me what these things are for,...

The check engine light is telling me that its those things, so do I have to replace the sensors themselves or is the light coming on because the sensors are detecting something going wrong with my car!!!
Please someone help!
Joe.
PS>..Let me know if i should replace them and i will ASAP

 

Your Maxima has one Knock Sensor and three Oxygen Sensors.

The Knock Sensor is mounted on the engine block, in the "V" just below the intake manifold. It senses when the engine is knocking. Engine knock is also called "pinging". When the KS senses pinging it signals the Engine Control Module (the computer). The ECM responds by adjusting the spark timing to eliminate the pinging. This is important because pinging is more than an annoying sound, it is a symptom of a potentially damaging combustion condition.

There are two front Oxygen Sensors, one in each branch of the Y-pipe. They monitor the oxygen concentration in the exhaust stream of their respective cylinder banks. Signals from these sensors are used by the ECM to continually adjust the fuel injectors' duty cycle (their "on time") to maintain an ideal fuel-to-air ratio. The rear Oxygen Sensor is mounted downstream of the main Catalytic Converter. It's purpose is to monitor the performance of the Cat.

There is some ambiguity in the term "rear O2 sensor". Some people use that term to mean the one behind the Cat. Some people use it to mean the sensor which serves the rear bank of cylinders.

It's difficult to tell you whether you need to replace those sensors. The Malfunction Indicator Light (a/k/a Check Engine Light, or Service Engine Soon light) is telling you something is wrong. The ECM does its best to tell you what the problem is by means of stored Diagnostic Trouble Codes. However, the owner or technician has to use thought and judgement when evaluating those codes.

A DTC may indicate a problem with an Oxygen Sensor. That does not mean you have to replace the sensor. It could mean
(1) The sensor is dead and must be replaced.
(2) The sensor wiring is bad (open or short circuit).

If the ECM thinks the sensor is dead, how did that come to pass? Sensors can "go bad" from long use. However, they may also go bad because of an engine defect elsewhere. Examples:
(1) A fuel injector is sticking open, causing one cylinder to run extremely rich. That cylinder is emitting black soot, and that soot has collected on the sensing tip of the sensor. The sensor has become "blind" because of a crust of deposits. Replacing that sensor will get rid of the DTC for a while, but the DTC will come back because the basic problem was a bad injector.
(2) One cylinder is burning oil because it has a broken piston ring. The exhaust from that cylinder is really dirty and leaves gunk on the sensor and is also clogging up the Catalytic Converter. As explained above, replacing the sensor is not the right fix.

You need to have a skilled technician evaluate your engine and decide why those DTCs were set, and what to do about it. It's a mistake to just replace sensors without diagnosing and repairing the underlying cause.

 

Thank you VERY much mr. martin..
So do you recommend that i take my car right to the dealership and have them look for the problem?

Let me know.
joe.

 

I view the Oxygen Sensor and Knock Sensor as independent problems. The Oxygen Sensor should be fixed right away. The Knock Sensor fix may be postponed for a while.

Visit your dealer and ask if your car is eligible for a no-charge replacement of the Oxygen Sensor. There was a recall on some early 4Gen sensors. Some owners report a no-charge replacement even long after the car was off warranty. Your dealer can use your car's Vehicle Identification Number and do a computer lookup. That will tell him if your car was included in the recall, and also if the recall replacement was already done.

If the dealer won't replace the Oxygen Sensor at no charge, you may consider having some other repair facility do this work. This job can be done by any competent technician. In general, independents charge less than dealers. Ask your friends, neighbors, coworkers for recommendations. You might find an experienced Nissan dealer technician who has opened his own shop. That's a good man to know!

 

Hold on one minute!
The first thing that you should do, is to reset the ECU, and see if the code comes back. Sometimes it will, sometimes it won't.

My behind-the-cat 02 sensor was setting off codes about once every 3 weeks. The car was running fine, except a slight stumble at part throttle. With my NX 2000 I would have to clean the throttle body every 40,000 KM, or it would stumble just a little. So I had a look at mine, and WOW it was very dirty. I had about a 10mm ring of black crap around the TB.

I cleaned it, stumble gone, and it has been 12 weeks since the check engine light came on. (so far, so good)
There are 2 small holes just in front of the butterfly in the TB, and they are used by the ISC (idle speed control) They were either blocked, or partly blocked.

Why would that set off a trouble code? The car is not at idle.
WELL, as it turns out the ISC works up to about 3,000 rpm at part throttle to control how much air is bypassed the TB.

If you want proof, dis-connect your TB, and when you drive the car you will feel that the car has much less umph below 3,000 RPM.

ALSO, when the beshind-the-cat o2 sensor would trip a code, I would also have a knock sensor code. Since cleaning the TB, that code is gone as well.

My GUESS is that the ISC was not able to bypass the TB (which leans the mixture), the mixture got a little lean at low rpm (remember the old race saying "lean is mean" a lean mixture burns faster and MUCH hotter then a rich mixture) and that would encourage the motor to knock. The sensor would pick it up, and TADA!

Clean your TB, if it works for you GREAT. If not take it to the shop.

 

MaxVQ, I like all your ideas. Your input is based on actual experience and that carries a lot of weight. There is one part of your post which I don't understand. You said
If you want proof, dis-connect your TB, and when you drive the car you will feel that the car has much less umph below 3,000 RPM.
TB means Throttle Body, right? How can this be disconnected and still have the engine run?

I agree that pinging can be caused by a lean mixture. However, Diagnostic Trouble Code 0304 means the Knock Sensor has failed or the wiring is bad (open circuit or short circuit). I don't see how pinging would set DTC 0304. It is the function of the Knock Sensor to detect pinging and signal the Engine Control Module, not to roll over and die when pinging is encountered.

 

[/QUOTE]
I agree that pinging can be caused by a lean mixture. However, Diagnostic Trouble Code 0304 means the Knock Sensor has failed or the wiring is bad (open circuit or short circuit). I don't see how pinging would set DTC 0304. It is the function of the Knock Sensor to detect pinging and signal the Engine Control Module, not to roll over and die when pinging is encountered. [/I][/QUOTE]

Two things:

(1) When I said TB, I ment ISC. (ops!)

(2) You are right on the code 0304. I know the Black Bible (Nissan Factory Manual) say that the 0304 code means "signal out of normal range" which really is an open or short, BUT maybe if it slightly pinged a lot at low RPM, that *MAY* be interperated by the ECU as a problem.

I don't know, BUT I will say it agian "So far, so good"

Thanks for catch in my mistake.