Bad Knock Sensor Symptoms [Functions and Replacement Cost]

You are driving down the road, enjoying your favorite music playing on the car stereo, having a great time. The latest new-gen cars are made to be utterly quiet, and there is no way you can come to know what happens in the engine compartment. 

However, you notice a check engine light go on, distracting you. There is also a slight chance of hearing a faint knocking sound when you increase the speed while you are on the highways.

You may feel that something is not right, and you feel the importance of finding out the root cause of the situation. But, before you know the root cause of the situation it is important to know what is knock sensor is? A bad knock sensor could be the reason behind the unwanted noises emanating from your car. 

What Is A Knock? 

An engine sound is also called a ping or detonation; a knock occurs when an unplanned explosion happens inside a cylinder. The spark plug ignition starts a flame face that travels through the space in the cylinder. The movement of the flame creates pressure with the air and fuel mixture.

As the pressure increases, the temperature also increases and sometimes gets so hot that it starts one more ignition, another explosion. When both the flames collide, you get a knock.

An engine knock usually sounds like a knock, click, or ping, and you will notice the sound more when you accelerate. Getting a knock is not a good thing for your car. 

What Is A Knock Sensor?

The engines in modern cars have various sensors which control most of their functions. These include fuel management sensors, emissions sensors, etc. A knock sensor has a very important part of your car engine. It can identify any noises that are caused by detonation. 

Detonation in an engine is caused by a fuel explosion in the engine’s cylinders. Once the knock sensor identifies the detonation, it can make changes to the ignition timing accordingly.

When the knock sensor does not work properly, the detonation is not regulated correctly, leading to serious engine damage. It is important to identify the signs indicating a knock sensor is not working properly and replace it well to avoid more extensive repairs. 

What Can Cause An Engine Knock?

There are various reasons why an engine knock can happen. Some reasons could be: 

  • The spark plug is not igniting the engine at the correct time
  • Keep an eye on the fuel ratio, as an imbalance in the fuel ratio can cause many other problems
  • If there is dirt and other contaminants in the cylinder, they can be damaging
  • Sometimes, the wrong spark plugs or incorrect spark plug gaps can cause unwanted sparks. 

The Major Bad Knock Sensor Symptoms

The following are the main symptoms or signs of a bad knock sensor, and you should take this as a warning and carry out a replacement quickly.

  • You will notice a strange feeling when you accelerate the car or load it up with a lot of gear. You can test by starting the car and pressing on the gas. Check the tachometer for the RPM range. If you notice some stuttering and stalling, then the reason could be a bad knock sensor. 
  • Another common symptom of a bad knock sensor is if your car starts giving poor gas mileage. If you are careful about the mileage generally, you will notice it quickly. The car’s computer will change the timing and make the car run less efficiently after noticing a bad knock sensor. 
  • The check engine light is the simplest to decipher a bad knock system symptom. Many cars have a steady light, while some have a flashing light. You can check the car’s manual to see what the flashing light means. If you wish, you can take the help of an experienced mechanic, who can diagnose the problem quickly. 
  • All modern cars are very smart, and the minute they notice a faulty knock sensor, they slow down the timing. By doing this, there is prevention from further damage to the engine. This is done to prevent further damage to the engine or the car. 

Can You Replace A Bad Knock Sensor?

If you are keen and have some car repair experience, you can try to replace the sensor on your own. It may take approximately an hour. There are many videos on this subject available online, which can be helpful. If you are new to car repairs, take your time and remember to take a lot of pictures before starting.

This ensures that you know where everything goes once you are ready to put it back. Remember to disconnect the battery and other wires; otherwise, it may be dangerous.

Once you replace the faulty sensor with a new one, check the pictures you have taken and put back everything correctly. Now, reconnect the battery and check if the Check Engine Light is off. 


Q. Should you drive your car in times of emergency even if your sense something is wrong with the sensor?

A. You can drive with a bad knock sensor in an emergency, the car may perform badly and the engine may be damaged badly. If you are sure that the knock sensor needs replacement, it is best not to waste any time and do it immediately. 

Q. What will happen if you do not replace the knock sensor?

A. If you do not replace the knock sensor, even when it is not working, your car engine may start making noises without being detected. This may lead to reduced power, lesser fuel mileage, and many other problems. 

Q. How much does it usually cost to replace a knock sensor? 

A. It will mainly depend on the brand you choose, but the average cost of a knock sensor is between $ 300 to $ 400. The mechanic who fixes it may charge between $ 150 to $250 depending on the location and size of the garage. 


When anything goes wrong with your car, you may have to check all possible issues to determine the cause of the problem. Modern technology is so good that the car can detect the problem and inform you about it. The sensors in the car are super sensitive and monitor the functioning of a car thoroughly.

The sensors can catch even a slight tremor or jerk. Among the dozens of sensors in the car is the knock sensor, which keeps a close watch on all types of noises and vibrations. The knock sensor can prevent major damage to the engine and keep your car going smoothly. 

Liam Dare

As CEO of, my passion for the automotive world motivates me to build online businesses that provide information and entertainment to users. I am proud to contribute in a positive way to the automotive community.