Ok, so heres the deal. I am driving along normally, shifting below 3500 rpms for about 25 miles. The SES randomly comes on and I begin to get angry. I bought the car 6 weeks ago and I have only put 1200 miles on it since ive bought it. It currently has 108,935 miles and has appeared to run fine ever since I have bought it. I occasionally rev her up and let her loose, but I drive this way less than 10% of the time. So I am driving normally today after driving for about 30 mins (20 miles) and I look down and the SES light is on.

I bring it to Autozone and they told me that its P0325 (knock sensor) and P0420 (cat converter). Now the exhaust was recently replaced, but it only looks like the b pipe and muffler were changed cause the cat is all rusty. The engine has a slight ticking sound when idling but you cannot hear it in the car...or even outside the car unless the hood is up. My car is not running in safe mode, nor has it been very sluggish lately. I do not think that it is the KS but who knows.

I am thinking of resetting the ECU and clearing the SES but I do not know. Please help me out as this has been pissing me off all afternoon. Thanks!

 

KS will not make you feel sluggish. (not as sluggish as I think you are expecting). It is very easy to not even notice a bad knock sensor. If the coed is there, replace it. As for the cat, here is the problem. The only way to really tell is to take it down and inspect it. actually looking for a solid clogged cat or shattered pieces. There is a chance that the rear O2 is bad, an dif the cat is bad I would assume the O2 sensor is too. Do you have emissions testing in Boston? If so, that is another way to see if the cat itself is bad (although the front cats in the headers/downpipe may do enough to allow the car to pass). Its a tough call. Change the KS and see if you can get the cat inspected.

 

Haha the funny thing is the car passed emissions on Jan 20th. So I would of thought that maybe they would of hinted to me that the cat was bad or I was close to failing. Although its been 6 weeks, I think that I will try changing the KS and go from there. I just though that KS threw a ghost code and would not trip the SES (Code 0304). Maybe I was misinformed.

 

you are correct. but you have 2 codes so something else is wrong. either the rear O2 sensor, or the cat itself is bad. the front cat's will allow you to pass emissions (i think). change the KS, reset the ECU and see which code comes back. or just buy my rear O2 sensor brand new from nissan and install it.

 

What if I test the KS and see if its good? I was planning on just resetting the SES light tomorrow b/c I want to see how quickly it comes back. Other than running sluggish (which I dont even notice) what damage will a faulty knock sensor do?

Also, I do not think that its an 02 sensor only because it looks like the exhaust was replaced from the cat back (not including cat). However, Im not the most mechanically inclined person so I would not know.

 

testing the KS isnt very reliable. while it may test ok under cold temp. that doesnt mean it will work properly under the higher operating temperatures of a running engine. pretty sure the ECU knows what it is doing.

 

Why would you just check the KS, it's not like it's located in a easily accessible area, plus you have a 99 like myself. There's so much useless crap running in that area unlike 97/98 that it took a shop 3 hours to change my KS. I would just go ahead and change the KS, even if you don't fell any loss of power believe me it's there. Plus you gas mileage is suffering slightly.

 

Yeah gas mileage hasnt been the best...I saw a few weeks ago that someone was selling KS for $60. I will look back and see if i can find the post because there is someone on the org who says they will change the KS for free for 4th gens. I need to find a new OEM one for cheap...as I do not want to put some knockoff brand in my car.

 

nissanparts.org.... KS for $93 = the exact same one as the ones on Ebay. brownbox. thats why they sell 2. 1 for $90+ and the other for $140+.

 

My plan now is to reset the ECU tomorrow...and play it by ear. If the SES comes back on again, I will plan on changing the KS and then going from there. Thanks for the feedback so far guys.

 

I also notice that when I started the car today (3 times so far today) each time it starts up right away, but makes a loud click/knock right as it starts up? I think that its the knock sensor. Any ideas? This is the only noticable difference I have noticed since the SES light came on.

 

knock sensors dont knock.

 

I would reset the SES light and see what comes back. The KS is probably being tripped by the bad cat. So if both of those codes come back again replace the cat first and then reset the SES then get it checked again in like 50 miles and see if the KS code is back. If it is then replace the KS

 

how is a bad cat going to trip a KS code? They aren't connected.

 

I got the damn cover off the ECU, and broke the plastic covering over the screw hole and after fiddling with the screw for about 10 minutes, I still cant figure out how to check/reset the SES light. I know that you turn it clockwise till stop, wait 3 seconds, turn counterclockwise till stop and read code. Repeat 2x and it will reset. I cannot get my screwdriver on the head of the ECU. I am using a smaller eyeglass screwdriver also.

 

Dear "99BlackMaxMS,"

It has come to my attention that you are a tool. Secondly, I have received word via satellite that you were "revving her up, and letting her loose," as you would say, right before this light came on. I also received word that a character by the name of "Rizzle" came to the same location shortly after you. Not only did his Z "pwn" your Maxima, but he also revved her up and lether loose much more than you did. If you could confirm any of these statements, please do.

"There is no spoone."- The Matricks

 

I see some has too much time on her hands.

for a '99 you need a mini phillips. not a flathead.

 

Haha I go to school w/ that kid. I decided to put back the cover and try later as its 5 degrees outside and after 15 mins fiddling with the screw I gave in. Ill prob just bring it to autozone and have them reset it.

I talked to Dave B today and he said that it sounds like the main cat is bad because of P0420. He said that sometimes it throws the KS code and he said to change the main cat then see if the KS code is still there.

I have decided to wait and reset the ECU and go from there. I read on another site that code P0420 only applies to Cali spec Maximas. "But from what I've heard and my experience, is that the P0420 code is just a computer bug, and won't affect driveability at all. I thought the P0420 might be related to the car running like crap, but it wasn't. It was all the MAF. I just unhooked the battery for a couple hours and the code went away."

 

autozone wont reset CEL's just read them.

 

I called them and they told me that they could clear the codes as long as you fixed something in the car and the codes were not going away. Ill just tell them i replaced KS and an 02 sensor and they should reset it...unless I am mistaken.

 

most people replace there KS around 100k. I would do the resistor trick to bypass the KS, reset the ECU and see what comes up.

 

I brought the car to Autozone again today, and after showing them a piece of paper w/ a knock sensor on it (supposobly a reciept to show proof to fix car) they reset the SES light and after driving 20 miles, its not back yet. Hopefully it wont come back, but well see.

 

bad or plugged cat can cause almost a backfire expecially on startup, which may cause a knock and trip the KS, good advice to check/replace the cat first.

 

Yeah, on the cold mornings the last 3 days, the car has made one distinctive knock right as it starts up. However when the car is warm, there is no knock. I am hoping that it is what tripped the SES light, and it will not come back.

Also thought I would add that when I got the car, my mechanic told me that there was a broken bracket on the y-pipe...I am not sure if this has anything to do with it tho.

 

incorrect. the KS code indicates a bad KS (or lack of one).

so unless your "backfire" actually breaks the KS because it has knocked so hard (rediculous)

.....Ish Dont Think So.

 

knock sensor is just that, it will throw a ghost code when the engine knocks, not saying the the KS is toast, a KS can throw a code once, ECU be reset, and the KS never throw a code again. Just a better idea to check the cat 1st

 

the knock sensor does not throw a code when it senses a knock. and a "ghost code" simply means that the code is present but it doesnt set off the CEL. just because you reset teh CEL doesnt mean teh code hasnt come back. the knock sensor senses engine knock and tells the ECU which in return alters the timing to compensate. If you get a KS code it is not becuase a knock was sensed. It is because the sensor itself is bad/unreadable.

Kind of like how a MAF Sensor doesnt throw a code when air flow is sensed. It throws a code when the MAF Sensor itself is not working or unreadable.

 

ok sorry dude I misunderstood the KS I guess, not enough info out there on the actual functions, but either way IMO I would check the cat first, and check for a KS ghost code after fixing the cat.

 

Thats what I plan on doing. If the CEL comes back, I will go back to Autozone and have the codes read. If code P0420 (cat) and P0325 (KS) come back then I will have the cat looked at first.