Blog Car Care Advice What’s a Knock Sensor? (+Symptoms of a Bad Knock Sensor)
Car Care Advice

What’s a Knock Sensor? (+Symptoms of a Bad Knock Sensor)

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Your car contains an array of sensors, from a knock sensor, a throttle position sensor, or yaw rate sensors.

Although the knock sensor isn’t as well-known as some of its counterparts, it’s crucial for keeping your car’s engine running right. It informs you when there’s detonation or pre-ignition.

But what happens when the knock sensor fails
What causes a faulty sensor?

In this article, we’ll cover what a knock sensor is, the symptoms of a bad knock sensor, and the causes of a faulty knock sensor. We’ll also answer some knock sensor-related FAQs.

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Let’s get started.

What Is a Knock Sensor?

Harmful forms of combustion like pre ignition and detonation typically cause engine knocking. 

The knock sensor is a piezoelectric sensor located outside the engine block. It records the knocking noise in all engine operating states to prevent engine damage. In other words, it detects high knock frequency vibrations of an engine and sends an electrical signal to the engine control unit (ECU).

But how exactly does it do that?
Well, the piezoelectric element inside the sensor detects the engine’s knocking vibrations (mechanical energy) and turns it into electrical energy. The electrical energy generated is called the sensor output voltage. 

Why are these vibration detection sensors important?
The engine knock sensor is necessary because even though loud knocking and pinging are audible to humans, the sensor detects imperceptible noises and irregular vibrations. 

So when the sensor detects any high frequency engine vibration characteristic or knocking noise, it sends an appropriate voltage signal to the engine control unit. Once the ECU receives the voltage signal, it retards the spark plug timing. 

Simply put, a signal from the knock sensor helps the ECU know to reduce the pressure and maximum temperature inside the engine, and retard the engine timing— to stop the knocking

As a result, a knock sensor can help reduce fuel consumption, improve fuel economy, and increase torque.

But what happens when you have a bad knock sensor? 
Let’s find out!

What Are the Signs of a Bad Knock Sensor?

Knock sensor failure can easily cause one or more noticeable symptoms. 
The most common sensor problems include the following:

1.  An Illuminated Check Engine Light

When the Powertrain Control Module (PCM) detects a faulty knock sensor or voltage circuit, it will turn the engine light on and trigger a related diagnostic trouble code (DTC).

2. A Pinging Noise From the Engine

In case of a knock sensor failure, the PCM may not be able to recognize or fix the spark knock frequency. The failed sensor can result in a metallic pinging noise from the engine. 

You’ll also notice that the noise gets most prominent when the engine is under a heavy load.

3. Bad Engine Performance

A malfunctioning engine knock sensor can cause the PCM to wrongly adjust the ignition timing, resulting in inadequate engine performance. The engine may not feel right while driving at high speed or when the vehicle is carrying a weighted load.

In such a case, you should get it checked out by a mechanic even if the check engine light isn’t on.

4. Poor Acceleration

A faulty knock sensor may not let the engine properly accelerate while driving on the highway, causing the car to lose fuel mileage (reduced fuel economy). 

5. Power Loss

Once the engine control unit realizes the knock sensor isn’t working correctly, your car will likely lose power. The amount of lost power will depend on the octane limit of the engine and how heavily it depends on the knock sensor input. 

The cars with the most power loss are high-compression and flex fuel engines. That’s because, until you repair the knock sensor, the power loss slows the engine timing and keeps the transmission out of drive.

Note: The slow timing lets you get to safety or get your car fixed but protects you from going very far.

You may wonder why you’d experience a knock sensor failure in the first place. 
Let’s find out!

What Are the Causes of a Faulty Knock Sensor?

There are multiple reasons for an engine knock to happen. 

Here are a few potential causes:

Now that you know everything about knock sensors, let’s answer some FAQs!

3 FAQs About Knock Sensors

Here are some answers to common knock sensor-related questions:

1. What Is Engine Detonation?

Engine detonation, also known as engine knock or ping, is the term for the spontaneous combustion of the remaining air fuel mixture in the combustion chamber — after normal spark-initiated combustion. 

Detonation results in knocking, causing damage primarily to the cylinder head gasket and cylinder head. 

2. Is It Safe to Drive With a Bad Knock Sensor?

With a faulty sensor, your car’s engine may produce higher emissions because it may run hot. 

The Environmental Protection Agency doesn’t permit such high emissions. That’s exactly why the decreased acceleration safety feature has been designed.

3. How Do I Test a Knock Sensor?

Before opting for a knock sensor replacement, testing a suspected sensor is best.  

Note: The following are general guidelines. Please consult your car’s manufacturer information for specific repair instructions and recommended safety procedures. If you’re not very handy with car parts and repairs, it’s best to contact a professional mechanic.

Here’s how to quickly test knock vibration detection sensors:

1. Check for Diagnostic Trouble Codes to narrow down the troubleshooting process using a scan tool or code reader. 

2. Perform a visual inspection to inspect wire damage and poor connections. Ensure that the knock sensor’s electrical connector is tight and clean.

3. Repair the issues you find during the visual inspection, and then clear the DTCs to check if the problem returns.

4. Test the knock sensor directly. 

PS: There are two types of piezoelectric knock sensors:

Tuned sensors amplify the voltage signal but only around the desired knock signal frequency range. The broadband sensor picks up vibrations and converts them into an electrical signal.

If your car has a wideband piezoelectric sensor, you can tap on the engine near the knock sensor. You’ll know if the sensor is working if it responds to the vibration.

But if you have the newer, resonance-style sensors, a professional will test them by forcing the engine to ping (produce a spark knock) while monitoring the sensor’s output signal.

Final Thoughts

A malfunctioning knock sensor isn’t the most common issue a driver may encounter. But when there’s a faulty sensor, you’ll need to repair it ASAP. A repair ensures you detect ignition timing problems, keep your fuel mileage up to par, and prevent engine damage.

Other engine problems share symptoms like a lit engine light or poor performance. So, the repair and test won’t be easy.  

As such, it’s best to let professionals like AutoNation Mobile Service handle a faulty knock sensor diagnosis and repair. 

AutoNation Mobile Service is a mobile auto maintenance and repair solution with an easy online booking process.
Contact us if you need a knock sensor repair or have other ignition issues. Our qualified mechanics will come to your driveway, and ensure your knock sensor detects every knock. If your throttle position sensor, or any other sensor has problems, we can help you resolve those, too!