Testing the Knock Sensor (KS): - The KS MUST be checked with a digital ohmmeter. Analog
ohmmeters may cause damage to vehicle. |
1) Raise the hood. View the engine from the driver's fender. Look into the deep valley between the
cylinder banks and below the intake manifold. Identify the KS as a black item fastened to the block
by a single vertical bolt. A wire harness wrapped in black leads toward you, out of the valley. That
is the KS sub-harness.
2) Follow the KS sub-harness to it's nearest connector. This is connector F121. It is located near the
upper right-hand corner of the valve cover of the forward cylinder bank, as viewed from the front of
3) Disconnect F121. You have to do a "press the latch and wiggle and pull" to disconnect it. F121 has only
two pins; if you see more than two pins, you have the wrong connector. Use a digital ohmmeter capable
of measuring more than 10 Megohms. You want to measure the pins of F121, not the sockets of the
matching connector. Measure the resistance between a good ground (such as the battery negative terminal)
and pin #2 of connector F121. On my car this is the highest of the two pins, the one closest to the
front of the car. The factory spec is 500 - 620 Kohms.
More on the dreaded KS:
Knock Sensor Diagnosis, Test and Installation
Once and for all, a bad knock sensor WILL NOT trip the check engine light. You must extract the codes from
the ECU to determine if your sensor is (potentially) bad (code 0304). Some other hints as to a bad KS: poor
acceleration below 3000 RPM (I was getting tired of being left behind at stoplights by Honda Civics),
and reduced gas mileage.
If you don’t already own a Haynes or Chilton manual, get one. In addition to telling you how to extract
the codes and locate the KS, there is a wealth of useful information for the do-it yourselfer. I am very
cheap, but this an area well worth spending the $20 or so (I bought mine off of EBAY for $10 shipped).
The ECU is located behind the center console on the drivers side. It is accessible by removing the plastic
cover panel right next to the gas pedal. The ECU is a metal box with a screw on the side covered by a piece
of tape. Peal back the tape to get at the screw. With the ignition in the “On” position, turn the screw all
the way clockwise, hold for at least 2 seconds, turn it all the way counterclockwise. Your CEL will now
start flashing out any codes stored in the ECU. The codes are two digit numbers. The first number is
signaled by long flashes, there will be a two second pause, and the second number will be indicated by a
series of short flashes. Long-long-long-pause-short-short-short-short = 0304 (KS fault). There may be more
than 1 code in the ECU, they will be flashed out sequentially and the whole sequence will repeat. To clear
the codes from the ECU, follow this sequence: clockwise, hold, counterclockwise, hold, clockwise, hold,
A good KS will measure ~550 k-Ohm resistance between the left pin of the KS and ground. You can check the
resistance without removing the KS by following the harness to the connector. The connector has two pins,
only one of them hooks up to the KS, so try them both. The act of whacking on the KS with your wrench can
(temporarily) start it working again, so checking the KS once you removed it may not give you the bad
reading you suspect. Hint: the connector is not the green one closest to the KS that gets in the way of
sticking your hand into the engine cavity (1995-1996 models – most likely 1995-1999). The correct
connector is a few inches closer to the front of the car. A heat shielded wire leads into the bottom of
it and there are two wires coming out the top, one clear and one black.
Low cost knock sensor sources: The dealer will charge you $160 or so for the sensor. I purchased a Nissan
OEM sensor off of EBAY for $90 shipped. I have seen them for as little as $80 and routinely for $100-$110.
Search for both “Maxima knock” and “Nissan knock” to make sure you get all the possible matches. You can
get a Bosch sensors for $115 shipped from http://www.nissanpartstore.com/nissan_knock_sensor.html
Two additional articles on KS replacement:
Modifications to the Skippynet method:
The article suggest using a 14” (or longer) ¼” socket extension to reach the KS retaining bolt. I found
that using a 9” extension made it much easier to get the u-joint onto the KS bolt. To save money, I used
two 3/8” drive extensions with a 3/8 to ¼ adapter, a ¼” drive u-joint and a ¼” 12mm socket (most of which
I already had). Skippynet suggest using a 12mm flex socket, but being cheap I used a separate socket and
u-joint so that I would have a more useful general purpose tool left over than the 12mm flex socket
(better yet, I borrowed the u-joint and saved another $8). A 3/8” u-joint is a slightly bigger animal and
in my opinion would make it hard to get the socket onto the bolt. It was not at all difficult to loosen
the bolt using this setup. I had also borrowed a 16” extension and found it very difficult to get the
socket onto the bolt head (I gave up after about 2 minutes, so it probably could have been done).
Once the bolt was out, I used a magnetic pickup to retrieve it (and eliminate the possibility of
dropping it into the engine cavity where as you will find out soon, is not a very friendly place). With
the bolt removed, you can pull the knock sensor out of the cavity by the harness and replace it. Next, I
put the bolt into the new KS and used a small piece of tape to hold it in place while I stuffed it back
into the engine cavity (again to eliminate the chance of dropping it in there).
Now the difficult(ish) part: I don’t see how you can locate the bolt into the hole and get it started
without stuffing your hand under the manifold. Others have done it using a claw type pickup to position
the bolt so no harm in trying. If you have very large hands, you might try enlisting a girlfriend, your
mom or a small child as others have done. I was able to get my hand in there and get the bolt hand tight,
then it was just a matter of using the 9” extension/u-joint setup again to do the final tightening. Your
hands will get cut up a little doing this, but the term “bloody stump” that gets thrown about is quite an
I was able to replace the KS in both cars in less than 1 hour including test drives using this method.
Post installation I got back the power of my VQ and gas mileage increased by 2 MPG.
The real key is using the skippynet socket extension method to crack the bolt loose. Once you have jammed
your hand into the engine cavity you will realize how difficult it would have been to get a wrench on the
bolt and loosen it.