Knock sensor what does it do ?

Check engine light on ? What code ? There are many different (DTC'S) - diagnostic trouble codes that can set for knock sensors ! There all don't mean that you replace the sensor .
DTC P0324: Knock Sensor (KS) Module Performance
DTC P0325: Knock Sensor (KS) Circuit Bank 1
DTC P0326: Knock Sensor (KS) Performance
DTC P0327: Knock Sensor (KS) Circuit Low Bank 1
DTC P0328: Knock Sensor (KS) Circuit High Bank 1
DTC P0330: Knock Sensor (KS) Circuit Bank 2
DTC P0332: Knock Sensor (KS) Circuit Low Bank 2
DTC P0333: Knock Sensor (KS) Circuit High Bank 2

The knock sensor (KS) system enables the control module to control the ignition timing for the best possible performance while protecting the engine from potentially damaging levels of detonation. The sensors are located below each cylinder head on the engine block. The KS produces an AC voltage signal that varies depending on the vibration level during engine operation. The control module adjusts the spark timing based on the amplitude and the frequency of the KS signal. The control module receives the KS signal through 2 isolated signal circuits. The control module learns a minimum KS noise level at idle and uses calibrated values for the rest of the RPM range. The control module should monitor a normal KS signal within the noise channel.
Knock Sensor Replacement - Bank 1
Remove the exhaust manifold lower heat shield. Refer to Exhaust Manifold Lower Heat Shield Replacement.
Disconnect the engine wiring harness electrical connector (1) from the bank 1 knock sensor (2).
Loosen the knock sensor bolt and remove the knock sensor.
Knock Sensor Replacement - Bank 2
Disconnect the negative battery cable. Refer to Battery Negative Cable Disconnection and Connection .
Remove the engine oil filter. Refer to Engine Oil and Oil Filter Replacement .
Lower the vehicle.
Disconnect the engine wiring harness electrical connector (2) from the bank 2 knock sensor (3).
Loosen the knock sensor bolt and remove the knock sensor.

its on all cars, that have it , close to the combustion, area.
its just a microphone (sonic) on side of the engine block.
Varies by actual engine in car, there can be many.
no country or engine or VIN stated.
in general......
it hears KNOCK, (aka. ping, or detonation , all very bad)
if you get DCT fo this (you failed to post the actual number.)
not posting the number makes for big time failure for any helper.
1: wires fell off sensor
2: sensor hit by road flying objects and its damaged.
3: horrid KNOCK, is it? knocking bad?
4: horrid mechanical damage making, clanking sounds, rod bearing going out, or? piston cracks or breaking up.

it's a microphone.
its either dead or intermitant, or hears something very wrong.

Should be on the side of the block facing the firewall. It may or may not be contacting coolant so you may have to drain the radiator.
Keep in mind knock sensors almost never fail. You may have an engine component making a noise or vibration the sensor is picking up.
The sensor is supposed to sense spark knock and tell the computer to adjust the spark timing down.

You may need to find a different shop

Nothing showing up, is about as common
as any vehicle coming into a repair bay

They know you won't pay diagnostic time or believe
anything your told,so they sent you on your way

It does sound like the sensor may be bad, but you also need to check the wiring.
This information comes from:

http://www.obd-codes.com/p0325

P0325 OBD-II Trouble Code Technical Description

Knock Sensor Circuit Malfunction
What does that mean?

A knock sensor tells the engine computer when one or more of your engine's cylinders are "knocking", that is, exploding the air/fuel mixture in a way that delivers less power and can be damaging to the engine if prolonged. The computer uses this information to tune the engine so that it doesn't knock. If your knock sensor wasn't operating properly, so that it was always indicating a knock, the engine computer's response may have been to change the ignition timing on your engine. Knock sensors generally bolt into or thread into the engine block. This P0325 code may show up intermittently or the Service Engine light may be lit constantly. Other knock sensor related DTC codes include P0330.
FB.init("dd7d9e9681341cde77587bc6a2029f6f"); OBD-Codes.com on Facebook Symptoms

You may notice drivability problems including a lack of power and perhaps some hesitation. There may also be other symptoms.
Causes

A code P0325 most likely means one or more of the following has happened:

• The knock sensor is faulty and needs to be replaced
• There is a wiring short/fault in the knock sensor circuit

Possible Solutions

• Check the resistance of the knock sensor (compare it to factory specs)
• Check for broken/frayed wires leading to the sensor
• Replace the knock sensor

i honestly have never seen a knock sensor go bad. a knock sensor detects spark knock,which is from low octane gas. if it hears this then the ecm retards the timing to stop the knock. you wont be able to hear the knock that its listening for. its like a little microphone stuck in the block. before you replace it you might want to check the plugs on the sensor itself. if they are corroded they could cause this problem.. ANYTIME you have engine codes ,always check your ground cables on the engine and the battery. i work in the marine industry and you would be amazed all the codes you can get just from having a corroded battery cable..like i said , i have never seen a knock sensor go bad..

Most vehicles are designed to run on 87 octane, unless specified different in the owners manual. The knock sensors on your engine will detect an engine knock and advance or ****** the timing accordingly. If your engine is spark knocking, your "Check Engine" light should be on detecting a faulty knock sensor, hope this helps, Gary

It's the valley of the engine underneath the intake manifold. It's about an $800 job because the top of the engine has to come off. However, unless you have a supercharged xterra it shouldn't effect performance. Also, the knock sensor will not trip the check engine light, as it does not indicate a primary problem, so if your light is on you should have another code that should be fixed first.

Well, you have another issue besides a knock sensor problem. A knock sensor code WILL NOT illuminate the check engine light. Check for other codes stored in the ECU. I have seen oxygen sensor issues cause knock sensor issues due to inaccurate fuel delivery resulting in engine ping at all times (causing the ECU to think that the knock sensor is bad because it's reporting ping at all times) - but also had codes for one or more O2 sensors.

Nissan Scope Technician- 10 years with Nissan

Check the knock sensor.

The light comes on when you climb a hill, accelerate quickly or do anything that advances the timing a lot and the knock sensor doesn't hear the knocks (pinging). The engine timing is not retarded, so you get a misfire.

The computer turns off the CEL if it sees the car start and run with no misfire of this nature 4 times in a row.