Random Car Facts: Engine Knock Sensor

Yes, you guessed it, the knock sensor is what determines whether or not your car is efficient at telling knock knock jokes. Yes, I know that was corny but why not??

So, what really is the knock sensor? It is a sensor mounted on the outside of your engine that measure vibrations.

What is the combustion cycle?

Before we get more into the knock sensor and its functions, let’s first discuss some of the things going on in that black box under your hood (also known as your engine). The parts of the engine that usually cause these knocks and are, consequently, most affected are the cylinders, pistons, and valves.

During a normal combustion cycle, an air/fuel mixture is released into the engine cylinder while the piston is in its lowest position. The piston then moves up the cylinder wall compressing the air/fuel mixture.

In a gasoline car, a spark plug ignites this compressed mixture. In a diesel car, the pressure from the compression causes the air/fuel mixture to ignite without a spark.

Once the mixture is ignited, it then forces the piston back down which transfers power to the drivetrain and allows you to go zoom zoom.  The piston is then driven back in the upwards position to push the remnants of the mixture, commonly known as exhaust, out of the engine.

Inline_6_Cylinder_with_firing_order_1-5-3-6-2-4 (1)


Now that you have an understanding of the system that is most affected by engine knocks and monitored by the engine knock sensor, let’s explain what engine knock actually is.

What is Engine Knock and How is it Caused?

Engine knock is a very common noise that can be easily ignored but can cause major problems. There are three main reasons you have engine knocking.

Wrong Octane of Fuel

Some cars, especially higher end cars, are designed to accept a certain octane of fuel (83,87,93). This octane level refers to the flash point of the fuel. In other words, it represents how easy it is for the fuel to burn.

Some might think it’s not a big deal to use the cheapest grade of gasoline (shoot why wouldn’t you with the price of premium in some places). These cars are pre-programmedAint nobody got time for that to have a certain mixture of fuel and air in the cylinder and are calibrated to account for the specific octane called out in the owner’s manual. When this mixture is not correct it can cause either premature combustion or delayed combustion. This is known as detonation when you have premature combustion which is what makes the knocking occur.

To avoid this issue, it is very important you follow the directions of your user manual when fueling your car.

Residue from Combustion

Gasoline is a carbon-based substance made up of chemicals that can leave behind built up carbon residue overtime. In other words, the gas leaves trash. Gasolines usually have a cleaner mixed in with it to prevent carbon build up.

This carbon build-up affects how the air and fuel in the cylinder compress. With the difference in compression, you experience the same issues that occur when using the wrong octane fuel (detailed above).

This issue is also more common in vehicles that use lower grade gasoline, such as regular unleaded. The lower grade gasoline does not burn as well as the higher-grade gasoline. However, it can be remedied by routinely using a fuel additive that cleans the carbon out of your combustion system.

Wrong Spark Plugs

The spark plugs are what begin the combustion in the cylinders. Every car is designed to use a specific spark to ignite the air/fuel mixture. Engine knocking can occur if the spark plug provides too much or too little of a spark.

Not only is the specific spark plug important but also the gap of the spark plug. The gap of the spark plug affects the type of spark produced by the plug.

Spark Plug

The Knock Sensor and Its Uses

The knock sensor is programmed to read a certain level of vibration which is then processed by the PCM, (powertrain control module) the brains of the car. If the vibration exceeds the sensor’s preprogrammed magnitude for vibrations then the timing of the sparks is adjusted to compensate for the irregular ignition. This time adjustment lowers the chance of detonation problems.

The sensor is made up of a crystal material that conducts electricity when pressure is applied to it. The knock sensor is programmed to have a certain voltage level but when that level goes up the PCM detects an issue and adjusts.

What Happens with a Bad Knock Sensor

Just like with any other sensor, this sensor can also malfunction. The symptoms of a malfunctioning knock sensor can sometimes be difficult to diagnose.

The most common symptoms are sluggish acceleration. As stated above, when the sensor detects too many vibrations, it adjusts the ignition timing. This, in turn, causes a much less responsive gas pedal.

This symptom will be more noticeable at highway speeds or during a hard acceleration.

Another symptom of a bad knock sensor is decreased gas mileage. When your ignition timing is slowed down, your car can no longer burn fuel efficiently lowering your fuel economy.


The knock sensor is very important and plays a key role in keeping your engine running.  Without the knock sensor, you risk damaging your cylinder walls, the pistons within your cylinders, or your valves.

If you suspect you are having a problem with your sensor, it is important to take your car to your trusted mechanic to look into the issue. Common knock sensor diagnostic codes are P0325-P0332.

Also, if your knock sensor is correctly reading engine knocks it is best to take your car to the mechanic to immediately address the issue to avoid further damage.

If you have any questions or comments drop them below or send us a message here. We’d love to hear from you. And remember – knowledge is the key to a fair repair.

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