The Vehicle Lab is reader-supported. If you decide to buy through a link on this page, we may earn an affiliate commission.  Learn More

Knock Sensor: Functions, Bad Symptoms, and Replacement Costs

The engine of your vehicle is controlled by a number of sensors used for regulating things like emissions, ignition timing, and temperature. One particular sensor is called a knock sensor. The term “knock” is also referred to as pinging, pinking, detonation, or spark knock.

The knock sensor is essentially a listening device that sits close to the engine, either on it, on the cylinder head, or on the intake manifold, and detects vibrations caused by an engine knock.

The sensor itself consists of a piezoelectric crystal and a resistor. These piezoelectric elements generate a voltage when pressure or vibration is applied to them. Due to the vibration, the counterweight in the sensor applies this pressure to the crystal. This pressure results in an electric charge which is the output signal of the sensor to the engine control unit (ECU).

The Function of the Knock Sensor

A car’s control module uses a computer to optimize the ignition timing of the engine. That, in turn, impacts mileage, power, and engine longevity. At high engine speeds, for example, the Control Module would advance the ignition timing. This would allow the air-fuel mixture in the engine adequate time to burn.

However, if the ignition time has advanced beyond a certain point, it could lead to pre-ignition. This uncontrolled ignition is called a knock. These tiny explosions can adversely affect engine performance. Luckily the knock sensor can detect the vibrations from the pre-ignition, send a signal to the ECU and delay the spark timing to avoid detonation.

What is a ‘Knock’?

Under normal circumstances, the Engine Control Unit (ECU) triggers a spark to ignite the air-fuel mixture in the combustion chamber of an engine. Since the mixture takes time to burn, the ECU adjusts this ignition timing in order to provide a balance of power and efficiency. This is known as a spark advance and retard.

Any delay would result in decreased power. Any advancement would cause self-ignition within the chamber. If left unchecked, it could even damage the engine permanently.

Symptoms of a Knock Sensor Malfunction

corroded knock sensor

We have known a knock sensor malfunction to cause extensive damage to an engine. We recommend that you look out for any of the following symptoms, should your vehicle display any of them.

The Check Engine Light (CEL)

This is one of the first symptoms of a bad knock sensor: The CEL is visible on your dashboard. There are a variety of reasons that could lead to the CEL lighting up, and a faulty knock sensor could be one of them. If the knock sensor detects an abnormality it will also light up the CEL.

Thumping Sounds from the Engine

As results of pre-ignition (knock), these sounds can be fairly loud. The source of these sounds are small combustions from the air-fuel mixture at the incorrect timings. These sounds will get progressively louder if the issue isn’t rectified.

Poor Acceleration

Since the knock sensor can no longer aid the ECU, the engine’s performance suffers. Hence, the car cannot accelerate at its normal responsiveness. The pressure created within the combustion chamber of the engine is often only a fraction of the engine’s capacity. As a result, the torque is lower and your car won’t accelerate properly.

Low Gas Mileage

A variety of factors could cause reduced mileage. However, if found in conjunction with some of the above, the knock sensor is most likely to blame. The ignition timing impacts more than power. When inefficient, it can cause unnecessary ignition or a knock, which leads to lower gas mileage.

Jerks and drags

The lack of a properly functioning knock sensor, if left ignored, can lead to permanent engine damage. The car could start to jerk and drag. It might even be accompanied by a burning smell. These are clear signs of permanent damage to the engine.

Knock Sensor Repair Cost

In most cases, you aren’t going to be able to repair a knock sensor on your own. You should also note that the cost to replace a knock sensor is going to vary by region and who you go to.

However, you can expect to pay between $200 to $400 to replace a knock sensor.


Ultimately, the knock sensor is vital to the proper functioning of your vehicle. Albeit a small component, the sensor protects the engine from internal damage. Renowned car manufacturers such as BMW and Chevrolet provide easy replacements for it.

However, a car’s engine is considerably more expensive than a knock sensor. So, we would recommend that any of the aforementioned symptoms must be taken seriously. Lastly, you likely won’t be able to replace this on your own, so visit a mechanic to have it done properly.


Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.

Leave a Reply

Your email address will not be published. Required fields are marked *

Copyright © 2020 is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for website owners to earn advertising fees by advertising and linking to amazon(.com,, .ca etc) and any other website that may be affiliated with Amazon Service LLC Associates Program.

linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram