What happens when the knock sensor goes bad?

Your car contains an array of sensors. One of them is the knock sensor. 

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.

This Article Contains:

• What Is A Knock Sensor?
• What Are The Signs Of A Bad Knock Sensor?
• What Are The Causes For A Faulty Knock Sensor?
• 3 FAQs About The Knock Sensor

Let’s get started.

What Is A Knock Sensor?

Engine knocking is typically caused by damaging forms of abnormal combustion like pre-ignition and detonation. 

The knock sensor, located on the outside of the engine block, records the knocking noise in all engine operating states to prevent engine damage. In other words, it listens out for the structure-borne vibration from the engine block.

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

So when the sensor detects any high-frequency engine vibration characteristic of knocking noise, it sends a voltage signal to the Electronic Control Unit (ECU). The Electronic Control Unit retards the spark plug timing after receiving the voltage signal. 

As a result, a knock sensor can reduce fuel consumption, help 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:

1. An Illuminated Check Engine Light

When the Powertrain Control Module (PCM) detects a faulty knock sensor or circuit, it will turn the check 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. 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 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 light or heavy load.

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

3. Poor Acceleration

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

4. Power Loss

Once the Engine Control Unit realizes the knock sensor isn’t working properly, your car most likely will 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 the power loss slows the timing and keeps the transmission out of drive til the knock sensor is replaced. 

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

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

What Are The Causes For A Faulty Knock Sensor?

There are multiple reasons for an engine knock to happen. 
Here are a few potential causes:

• The spark ignition isn’t occurring timely
• Improper air and fuel mixture ratio
• Deposits inside the cylinder (could be dirt, grime, and contaminants entering the cylinders)
• Faulty spark plugs
• Spark plugs that are of the wrong kind or with deposit buildup
• Spark plugs with incorrect spark plug gaps
• Low octane fuel
• Incorrect mounting of the knock sensors
• Mechanical damage
• Abnormally high engine operating temperatures
• Carbon buildup inside the combustion chamber or chambers

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

3 FAQs About The Knock Sensor

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 spontaneous combustion of the remaining fuel-air 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 To Test A Knock Sensor?

Before you opt for a knock sensor replacement, it’s best to test a suspect sensor. 

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:

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

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

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

• Test the knock sensor directly. 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 to be able to detect ignition timing problems, keep your fuel mileage up to par, and prevent engine damage.

Now, the repair nor the test will be easy. Some symptoms like a lit check engine light or poor engine performance are shared by other engine problems. 

So it’s best to let professionals, like RepairSmith, handle a faulty knock sensor diagnosis and repair. 

RepairSmith is a mobile auto maintenance and repair solution with an easy online booking process. If you need a knock sensor repair or have other ignition issues, get in touch with us.

Our ASE-certified mechanics will come over to ensure your knock sensor detects every knock!

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