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Knock Sensor

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Knock Sensor

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What is a knock sensor and where is it located?

A knock sensor, also referred to as a detonation sensor, works like a tuned microphone to pick up pinging or knocking in the engine. The sensor sends a signal to the car’s computer, which can adjust the ignition timing in response, to keep the engine running smoothly. In order to operate correctly, your engine requires that the air and fuel mixture inside the cylinders ignites at a very specific time. The knocking or pinging occurs when the fuel air mixture detonates at a time when it is not supposed to. This can lead to two flame fronts inside the combustion chamber, which create a shockwave when they collide. It’s also possible for incorrect ignition timing to create one poorly timed flame front that collides with the piston, which also causes a propagating disturbance called a shockwave. That shockwave is picked up by the sensor as a vibration or is heard by the driver as a sound that can range in sound and intensity from marbles hitting metal to an angry neighbor knocking on a door. 

To perform its role, the knock sensor must be attached to the engine, typically near the intake manifold.  Inline engines usually have one, while V-shaped engines will have two - one for each bank of cylinders.  The sensor contains a piezoelectric element (like what’s in a microphone) that is tuned to a certain vibration, particular to that engine. If that vibration, corresponding to knocking, occurs, a crystal vibrates, which creates a voltage in the sensor. The sensor then sends a signal to the computer, which modifies the ignition timing to avoid detonation. 

How do I know if my knock sensor needs to be replaced?

The knock sensor can fail over time due to corrosion of the electronics or general wear. If the engine knock sensor is not functioning, then the computer will not be able to account for knocking and correct the timing. The driver may notice the above mentioned pinging or knocking sounds. The sounds may be more prevalent during acceleration or at higher engine revolutions per minute. Knocking can cause damage to engine components such as pistons, valves, etc., and lead to costly repairs down the line, so it is best to solve this problem sooner rather than later. 

Many knock sensors regularly send a “no knock” signal to the computer. If the signal is not sensed at the computer, then the computer will operate under a fail safe mode. It will retard the ignition timing (as it would if knocking was sensed) constantly. This prevents knocking but makes for poorer combustion. This can reduce gas mileage and sap the engine of power. You may find acceleration slow or difficult. The lack of the “no-knock” signal will also cause the computer to activate the “check engine light.” A diagnostic scan tool will display codes ranging from P0325 to P0334 as shown below.

 Related OBDII codes

P0325 Knock Sensor 1 Circuit Malfunction (Bank I or Single Sensor)
P0326 Knock Sensor 1 Circuit Range/Performance (Bank 1 or Single Sensor)
P0327 Knock Sensor 1 Circuit Low Input (Bank I or Single Sensor)
P0328 Knock Sensor 1 Circuit High Input (Bank I or Single Sensor)
P0329 Knock Sensor 1 Circuit Intermittent (Bank 1 or Single Sensor)
P0330 Knock Sensor 2 Circuit Malfunction (Bank 2)
P0331 Knock Sensor 2 Circuit Range/Performance (Bank 2)
P0332 Knock Sensor 2 Circuit Low Input (Bank 2)
P0333 Knock Sensor 2 Circuit High Input (Bank 2)
P0334 Knock Sensor 2 Circuit Intermittent (Bank 2)

One way to check for yourself if the knock sensor is working is to tap the engine block with a rubber mallet with the engine running. The knock sensor should detect this vibration as knocking and signal the computer. You should be able to hear the engine RPM change if this is the case.

Can I replace a detonation sensor myself?

Even though the purpose served by the knock sensor is quite sophisticated, replacing it should prove relatively simple. The location of the knock sensor can vary from one model to another, but is found on the top of the engine, usually near the intake manifold. It may be bolted to the engine or the knock sensor itself may screw in like a spark plug. You will have to remove the old sensor, disconnect the wiring, and then connect the new sensor. It may be necessary to remove air intake parts or other nearby engine parts to access the detonation sensor. 

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