How to Test a Knock Sensor: Full Guide

Learning how to test a knock sensor is one of the most essential skills one should learn as a car owner. At some point, your knock sensor might get broken and you might need to replace it. However, the first thing you should do is test it and see whether it truly needs you to fix, replace it or everything is okay. Considering that you are here reading this, let's jump in and teach you how you can test a knock sensor without any effort!

What is a Knock and What is a Knock Sensor?

Knock and Knock Sensor

Before anything else, let's first define what is a knock. A knock, detonation, or engine ping is the sound that is generated when a second ignition within a cylinder occurs. The knock sensor is different from the spark plug, which provides the spark that is needed to ignite the air-fuel mixture.

In some cases, the pressure produced from the ignition is so high that it creates a second ignition - this results in the knock. Before learning how to test a knock sensor, it’s best to know what you’re dealing with so let's define what is a knock sensor as well.

A knock sensor is a car's “listening device”. It detects vibrations and sounds coming from the engine block. When the knock sensor picks up on these “knocks”, it sends electronic signals to the Engine Control Unit (ECU).

The ECU is a computerized module that determines whether the timing of ignition should be adjusted. Check out our other article that provides reviews of the best knock sensors available on the market. You can find it here.

Common Causes of Engine Knock

Causes of Engine knock

In learning how to test a knock sensor, it’s best to know what causes engine knock in the first place. Aside from your car's age, there are also other factors that can contribute to engine knocking. Here are some of the most common causes of engine knock:

1. High Temperatures

You might have noticed that your car moves slightly slower on hot days. This is because high temperatures increase the probability of combustion before the cylinder can even ignite the air-fuel mixture. This results in the poor timing of the spark.

2. Using the Wrong Fuel

Certain cars can run on multiple octane fuels. However, if the manufacturer specifies an octane for the car, using the wrong fuel could result in engine knocking. Therefore, you should avoid pumping fuel with a lower octane rating into your gas tank.

3. Carbon Buildup and Other Deposits

Throughout the time your car will accumulate a variety of deposits such as dirt, grime, and carbon buildup. All of these contaminants can create “hot spots” that trigger the uneven ignition of fuel. The good news is that this can be easily rectified through walnut blasting (learn more about it here) or using an air oil separator (AOS).

Top 5 Signs of a Broken Knock Sensor

Broken knock sensor

Keep an eye out for the following signs of a broken knock sensor. If you notice any of these, you should have your car checked by a professional mechanic immediately. You should also consider learning how to test a knock sensor so that you can diagnose the problem before it can escalate. Keep in mind that a knock sensor malfunction can result in permanent damage to your car's engine so you should always react quickly!

Check Engine Warning Light

engine warning light

One of the first signs of a faulty knock sensor is the check engine light (CEL). Otherwise known as the malfunction indicator lamp, the CEL can be found on your dashboard. It’s the yellow icon that lights up whenever a knock sensor malfunction is detected. When the CEL lights up or begins flashing, you should have your car checked immediately, as this could also indicate other problems.

Loud Thumping Sounds

Another sign of a broken knock sensor is a loud, thumping sound coming from the engine. The noise will slowly get louder if the issue isn’t fixed. Remember that the noise is produced due to problems with the ignition of the air-fuel mixture within the cylinder. Usually, the air and fuel would combust, but due to incorrect timing, it produces a thumping sound instead.

Low Gas Mileage

low gas mileage

While there are other reasons why your car is getting lower mileage, the knock sensor likely is to blame. If you’re the kind of driver that keeps an eye on your mileage data, you will probably notice when your miles per gallon starts to decline. When your knock sensor is inefficient, this can result in unnecessary ignition, and in turn, lower gas mileage. This is another red flag that sometimes might be wrong with the knock sensor of your car so you should not ignore it.

Poor Acceleration

Does your car struggle to accelerate no matter how hard you press the gas pedal? Chances are, your knock sensor is preventing your car from accelerating normally. This is because the knock sensor has stopped sending signals to the Engine Control Unit (ECU), the part responsible for the air-fuel combustion. To test whether your knock sensor is to blame, start your car, press on the gas, and keep an eye on the Revolutions Per Minute (RPM) range.

If the tachometer struggles to move, the problem is probably your knock sensor.

Jerking and Burning

car jerking

Engine knocking can permanently damage your car's engine. The worst sign of a knock sensor malfunction is usually accompanied by jerking or dragging motions and a burning smell.

The moment you notice any of these, it’s best to stop using your car, as any further use could result in expensive repairs. Remember, it’s cheaper to replace only the knock sensor than to replace the entire engine. To learn more signs of a broken knock sensor, check out our other blog post that deals with that in more detail. You can find it here.

How to Test a Knock Sensor: Step-by-Step Guide

test knock sensor

We already mentioned that knowing to test a knock sensor is the most important thing you should learn if you want to know whether something is wrong with it. Therefore, as we promised earlier, in this mini-guide we will teach you how you can do that all by yourself. So let's get rolling!

Step 1

Park your car on an even surface away from incoming traffic. Turn off the ignition and release the emergency brake. Open the hood and then turn the engine on again.

Step 2

Find the knock sensor. It is typically found on the engine block or intake manifold. The knock sensor will have a wire coming out of its top.

Step 3

Disconnect the wire from the knock sensor. You should simply tug on the base of the wire harness to remove it.

Step 4

Connect a negative multimeter to a ground point on the knock sensor, such as the battery terminal. The multimeter should show a reading that exceeds 10 ohms. This indicates that continuity exists. If continuity is not detected, the knock sensor should be replaced.

Here you will find detailed reviews of the best knock sensor according to our expert team. To see how testing a knock sensor exactly look like, check out the following video:


Unless you have the skills of a mechanic, you might not be able to repair the knock sensor by yourself. Replacement prices range from $200 to $400, but it’s best to have your car checked by a professional mechanic to make sure you don’t spend hundreds of dollars on the wrong part.

Don't forget that the engine is super expensive, so it’s best to have it checked by licensed mechanics who know how to test a knock sensor and replace it if necessary.

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