Knock Sensor

Knock Sensor
Knock Sensor

The engine management system in your vehicle uses information gathered from a wide variety of different sensors that monitor what is going on with the engine, one of these sensors is called the "knock sensor". This particular sensor is used in for the adjustment of ignition timing while the vehicle is being driven, to keep it at maximum efficiency and performance.

Older vehicles had distributors with centrifugal weights and vacuum advance cans that adjusted timing depending on rpm and how much throttle the engine has being given. Today’s vehicles use a computer (PCM) to adjust timing based on pre-programmed parameters within the computer as well as real-time info such as load, throttle position, engine rpm and vehicle speed that is being constantly gathered by the computer. The knock sensor enables the PCM to give the engine maximum timing to optimize performance while keeping it from pinging (detonation) which causes internal engine damage.

Detonation (Pinging) occurs when the air/fuel mixture ignites before the piston is in the correct position in its stroke; this is why it is also called “Pre-ignition”. When this occurs it causes the piston and rod assembly to force its way up through the combustion, which stresses the connecting rod, and can actually burn holes through the piston or cause it to break and can also cause premature engine bearing failure. Higher engine compression ratios require the use of higher octane fuels to help eliminate detonation, these vehicles usually will have a label near the fuel filler or in the manual letting you know what octane fuel is recommended for the vehicle.

The knock senor itself is basically a small microphone that the PCM uses to listen to the engine. It is designed to pick up sounds in the frequency range that detonation occurs, letting the PCM know that it needs to retard the ignition timing system. Different manufacturers will install this sensor in a wide variety of places, Some will only use 1 sensor for both sides of a V-8 or V-6 engine, others are centrally located in the valley under the intake manifold. Not all vehicles have a knock sensor in the system, so you should consult an automotive repair manual for the exact location of the knock sensor on your particular vehicle.

 If your check engine light comes on, and the code pertains to the knock sensor, you should first try using a higher grade of gasoline, some less expensive brands can cause detonation and set a code for the knock sensor, this may occur if the PCM has backed the timing down (retarded) as far as allowed, yet the knock sensor is still picking up detonation. Other internal engine issues can also set false knock sensor codes, depending on the noise they make.

When replacing the knock sensor in your vehicle always use a high quality OEM (Original equipment manufacturer) or equivalent replacement part. Less expensive parts tend to be made of inferior material and may not perform as well, fit correctly, or last as long as the high quality part will.

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35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-21)