Knock sensor circuit CEL

[Darius]

My girlfriend has a 96 altima and the CEL blinked off code "34" corresponding to the knock sensor circuit. What does this mean? Does it mean the knock sensor went bad or that there's knock? And does the ECU go into "safe mode" because of it?

Is there any way to get out of replacing the sensor by cutting the wire to it? I don't want to have to take the intake manifold off the motor to get at it...

[steve s14]

you may have a knock sensor code but that code will not cause the check engine light to come on. a blinking check engine light usually means you have a serious problem that can cause the catalytic converter to be damaged such as a misfire or something.does the engine run rough? if so, it's more likely to have a bad intake manifold gasket.

[Darius]

Originally, it threw a CEL for the front oxygen sensor. I replaced that and it worked fine for about three days, then the CEL started coming on sporatically even blinking irratically during acceleration. When it was blinking, the car was also hesitating slightly. I had that same problem with my Acura when my O2 sensor went bad. Replaced it and it ran fine afterwards. That is why I'm a bit confused on why this knock sensor code would be stored on the ECU shortly after changing the O2 sensor. Don't know if it's related at all or not??

It is a shielded circuit so I am wondering if something happened with the shielding becoming ungrounded and losing its effectiveness.

[steve s14]

i don't think it is related. i think the knock sensor code has been there for a while but you didn't know it until you pulled the code for the other problem.sounds like you have an intermittent misfire situation.check your spark plugs and wire set. if the spark plugs are ok, start the engine and spray the spark plug wires with a mist of water to get them damp.if you start seeing arcing or if the engine starts to run rough, the wires are bad. don't get cheap and buy aftermarket stuff. they don't last as long and most of the time, the parts don't fit as well.other things you can check that can cause a misfire are, vacuum leaks, intake boot leaks, or your distributor maybe going bad. it's not uncommon for the distributor to fail on that car at that mileage but i would make sure of that before replacing such an expensive part.

[Darius]

I'll check the plugs and wires. The distributor was just replaced like 10000 miles ago so it should be alright.

[Darius]

So the Alti started really acting up and vibrating roughly. The girlfriend made me drive it for a day and I pulled the wires individually. Cylinder 3 made no difference in idle when I pulled the plug.

So I started by replacing all of the plugs, which were in fairly old shape. The problem didnt' go away so I tested the resistence in each wire and they were within the specs of 4.5-5.6 kOhms/ft. So now I'm onto the distributor cap and rotor.

Does anybody have any pointers regarding the testing of these since I've never pulled one before. Hell I've done an entire RB swap, but they are distributorless

[NISTECH]

If your check engine light is intemittenly still flashing on accel you still have a misfire.A misfire as definined is the lack of combustion in one or more chambers.A lack of combustion can be caused by one of 3 thingsLack of sparkLack of fuelLack of compression

The last one would be constant and would not be intermitten.

This leaves you with 2 potential problems. Lack of spark or lack of fuel.

Spark is supplied by the plugs, wires,cap, rotor, and coil. The coil will only fire when the ECM tells it to, the ecm makes its decision to fire based off the crankangle sensor located in the bottom of the distributor. This sensor does not typically go bad unless it gets soaked with oil due to a bad seal along the sahft of the distributor. To see that area of the distributor you need to remove the rotor and the plate below the rotor. If you look in there and there is no oil, do not do anything else in there as that is likely not your problem.

Now for fuel you get fuel from the injector which gets fuel from the fuel line from the pump inside the fuel tank. The injector pulses fuel into the cyl when the ECM tells it to. The esm makes its decision to fire based on input from the crank angel sensor located in the distributor.

Typically when these distibutors fail the car flat out dies while driving. unless your having a die out problem I would not go after the distributor. The altima is known to suck the intake gasket at the #4 intake runner at the head. Though I have never seen the CEL flash when this happens.

My first instinct would be to check the ignition parts ,make sure they are nissan products and not after market garbage. The cap,rotor,wires and plugs should all be nissan OEM parts. They are basic ignition parts and should always stay OEM.

If that all checks out. Check for injector pulse with a noid light, check to make sure resistance is the same on all the injectors of a warm engine. Make sure you can hear them click.

[240sd]

I have a similar problem#34 code and she won't fire up at all...let me know what happens....thanks

[NISTECH]

Code 34[knock sensor] will not cause a no start incedent or cause a rough run condition. The effects of the knock sensor failing would be so minimal most would not know its failing.

THe code is because either there is a poor connection between the knock sensor and the harness that plugs into it or the plastic housing on the sensor itself is cracked.

[Darius]

I pulled the cap tonight and checked for resistance between the contacts and spark plug wire contacts...there wasn't even continuity between two cylinders. I don't know how this thing was still running, but I took the cap into the garage, chipped the corrosion off with a razor blade, sanded it with steel wool and put it back on. The motor runs quite a bit smoother than before, but it is still missing cylinder #3. The rotor looked a little rough/rusty, but if the other three cylinders are getting spark, I don't think it's too big of an issue.

I'm thinking it's an injector problem, so I'll listen to it and see if the 3rd injector is firing this weekend.

[steve s14]

does the engine run rough? if so, it's more likely to have a bad intake manifold gasket.

If your check engine light is intemittenly still flashing on accel you still have a misfire.A misfire as definined is the lack of combustion in one or more chambers.A lack of combustion can be caused by one of 3 thingsLack of sparkLack of fuelLack of compression

The altima is known to suck the intake gasket at the #4 intake runner at the head. Though I have never seen the CEL flash when this happens.

i personally seen a bad intake manifold gasket set a CEL for a cylinder misfire numerous times. check it again so you are not chasing your tail on this one.

[Darius]

Here's what I did last night:

Checked compression = 154 psi on a semi-warm engine.Pulled the plug on #3 (the one not firing) and grounded it to the valve cover. It had spark. Pulled the fuel injector to inspect for physical damage or wear, none to be found. I didn't have my multimeter with me at this point so I couldn't check the voltage at the injector harness or resistance through the injector.

I'm going to bring some starting fluid to the car tonight and spray some down by the intake manifold to check for leaks.

How difficult is it to remove the intake manifold from this motor with it still in the car? I haven't even checked how accessible the head/manifold bolts are. I'd like to do it myself because my girlfriend is broke and I'm a good boyfriend

[Darius]

Girlfriend's dad brought it into the shop before I could look at the fuel injector and that's what it turned out to be. I was wondering why she was still getting 27 mpg while her motor wasn't running on one-cylinder.

[NISTECH]

It wasnt putting any gas in that cyl. The other cyl's were not increasing it enough to make up for what wasnt being dumped in 3.