To say that modern engines are complex pieces of engineering would be an understatement. Almost every mechanical component is governed by a sensor of some sort, which is why modern engines are so efficient. 

Among them is the knock sensor — a crucial acoustic device without which your engine’s lifespan wouldn’t be anywhere near as long as it is now. But, what is a knock sensor? What is engine knock, and what role does a knock sensor play in preventing it? We will answer all these questions and more in this guide!

What is a Knock Sensor, and How Does It Work?

Internal combustion engines produce a lot of heat, either as a byproduct of the process or the motion it creates. Fundamentally, it’s a controlled explosion.

Engine knock is a phenomenon that occurs when residual heat in the cylinder ignites the air-fuel mixture before the spark plug does. Sometime around the 1920s, car manufacturers discovered that this did a lot of damage to engines. Engine knock was mitigated by adding lead to fuel. However, lead is toxic, especially in particulate form. Not only that, but it turns out that addition of lead to fuel caused one of the largest catastrophes in human history. Needless to say, once the full scope of damage was known, a lead had to go. 

What Is an Engine Knock Sensor?

New regulations forced manufacturers to phase out lead, so they had to find a new way to deal with engine knock. Their solution was the engine knock sensor.

Like any combustion process, engine knock gives off sound and vibration. Manufacturers have designed the engine knock sensor to detect these vibrations. It is like a stethoscope that is constantly listening for knock. The ECU receives the signal and adjusts ignition timing based on that. A byproduct of this adjustment is improved fuel economy and increased power lower in the rev range.

What Happens When a Knock Sensor Goes Bad?

Without a knock sensor, there is no way to control engine knock. The worst part is that although many cars will go into limp mode, knock sensor failure will not cause most pre-2000s to early 2000s engines to stop running when they fail.

What Are the Symptoms of a Faulty Knock Sensor?

The symptoms of a faulty knock sensor are all engine-related. Engine knock is something you don’t want to happen to you. Even though it’s a common occurrence, the consequences can mean the end of your engine if your knock sensor has failed.

Check Engine Light

A check engine light strikes fear into the heart of any car owner. However, it is a great tool to inform you that something is wrong with your car so that you can attend to it.

The next step in your diagnosis process should be to plug in your ODB II reader and get a list of fault codes. You should always stop as soon as possible when you get any check engine light.

Stacked Fault Log

Sometimes a sensor issue, particularly with a knock sensor, can lead to many other problems, like engine misfires. Your ODB II reader will usually show multiple faults, as one problem can cause others. It’s best to check the fault log of your vehicle before jumping to any conclusions.

Reduced Engine Performance

Engine knock reduces performance. The knock sensor solves this problem, and when it fails, you can experience acceleration problems. You might also experience excess carbon build-up and reduced fuel economy because of the disruption in ignition timing.

Troubleshooting a Bad Knock Sensor

Knock sensors don’t fail often. Manufacturers design them to withstand the high temperatures of an engine. Yet, these things go bad from time to time. Engine accessibility differs from car to car, but it’s usually going to be a tight squeeze to remove the sensor.

Although the knock sensor location differs between manufacturers, the knock sensor is on the outside lower part of the intake manifold or the top of the engine block. It’s a circular-shaped sensor with inner metal/ceramic rings connected to the wiring harness.

ODB II Reader

An ODB II reader will point you in the right direction almost immediately. The trouble code you’ll deal with is most likely going to be P0325 code. This code can mean a couple of different things. For one, it could mean that your ECU (Engine Control Unit) is bad. Although this is rare, it can occur and is the worst-case scenario. 

There are multiple error codes for a knock sensor, but for the sake of this article, we will only mention the main one P0325. 

Acoustic Test

In the same way that your knock sensor can detect high frequency, you can also perceive the sound that knock gives off. When stepping on the throttle, you will hear an audible pinging noise coming from the engine. The moment you hear engine knock is the moment you should pull over, kill the engine and call a tow truck. 

Listening to your engine is the least invasive way of testing a knock sensor, as the other methods require you to get your hands dirty and remove the sensor.

Visual Inspection

Manufacturers design knock sensors to withstand the high temperatures of the engine, but often, they compromise on the quality of wiring they use. Any damaged wiring can cause voltage or data loss. The placement of the knock sensor makes it difficult to inspect without removal. Make sure that if you disconnect the sensor, your car is not only off, but the power is not on.

Any damage to the sensor metal can also cause problems. Look for any major scratches or exposed parts. One thing to note is that depending on the type of engine, there can be more than one sensor. A V6, V8, or V12 will have two sensors, and most inline engines have one.

Resistance Testing

An easy way to test your knock sensor is using a multimeter. Most modern knock sensors are piezoelectric. So they will give off voltage based on mechanical interaction. 

Some recommend banging on the sensor to simulate the mechanical stress the sensor measures in the engine. However, this can damage the piezoelectric element of the sensor, which would defeat the purpose of testing it.

Test this sensor with no voltage flow by using a ground resistance test. Connect the positive (red) multimeter probe to the sensor and the negative (black) probe to ground. Ground can be any connection to the earth, but we recommend using the negative (-) battery terminal in your car.

Set the multimeter to AC to prevent damage to the sensor and set it to above 10 Ω. The sensor should output more than that.

You can also test the sensor while the car is on. Here, connect your positive multimeter probe to the back of the sensor connector, contacting the metal. The multimeter will read around 2KΩ to 3KΩ.

Voltage Testing

Once you’ve established that there is resistance, it’s now time to test for voltage. Testing for voltage in combination with resistance is a sure-fire way to ensure that your sensor works.

Turn the key to the accessory position, but don’t start the car just yet. Connect your positive probe to the back of the sensor, where the wires enter. This method is called back-probing.

This method allows us to test for voltage without damaging the sensor or the insulation. Depending on your knock sensor, there may be a cover that you have to remove to access it.

The reading will be around 5V when the connector is unplugged from the sensor. When the sensor is connected, this should drop to about 2.5V. You can also test it when the car is running, which should give you below 0.2V.

These numbers differ slightly from car to car, so it’s always advisable to consult your owner’s manual before testing or changing anything. They also vary based on the type of sensor. Although most are piezoelectric-based, some older cars still use a resistor-based sensor.

Oscilloscope

An oscilloscope is a great way to measure whether your knock sensor is doing its job.

In the same way, you tested for ground using the multimeter. You can also test resistance using the oscilloscope. Connect the probes from the knock sensor pin to the negative battery terminal (ground). Open the throttle using the pedal or valve. The graph should spike and remain high.

If the graph on the oscilloscope doesn’t rise, it can either mean no knock is occurring or your sensor is faulty. You can simulate engine knock by lightly tapping on the engine block using whatever tools you have. If there is no spike in the graph, your sensor needs replacing.

Torque

When you buy your car new, it’s unlikely that any bolt is under or over-torqued. However, parts age and corrode, and maybe you’ve conducted another repair or upgrade and neglected to fasten the bolt properly. It happens to everyone. You have to factor in torque when you remove and either replace or return the knock sensor to the engine.

You will need a torque wrench for this job as you will need to read the exact torque that the bolt is fastened at. There’s no compromising on this, as the sensor will not function correctly without the correct torque being applied. If you have your wrench, turn it until the screen reads 20-25 ft-lbs for threaded bolts and 15-20 ft-lbs for bolts without a thread.

Can I Drive With a Bad Knock Sensor?

Engine knock is a serious issue. Do not drive without an engine knock sensor or a bad one, mainly because it solves a fundamental problem with the gasoline engine. Leaving it to its own devices can lead to damaged pistons, o2 sensor failure, a damaged head gasket, and many other catastrophic engine issues.

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