5 Signs and Costs of a Bad Knock Sensor

Your vehicle has a lot of sensors. They perform many functions. One of the most important sensors in your vehicle's protection is the knock sensor.

What does a knock sensor actually do? How can you tell if yours needs to be replaced? And, most importantly, what is the estimated cost to replace it? We’ll answer all those questions and more in this comprehensive guide. Let’s take a look at the signs to look for first.

Check engine lights on the dashboard are the first sign that you have a bad knock sensor. In the most extreme cases, it can cause a detonating or knocking engine. However, you'll notice symptoms such as low engine power and fuel consumption.

Here is a list that explains the five most typical symptoms of a defective knock sensor.

Bad Knock Sensor Symptoms

1. Verify Engine Light

Just because you have a check engine light for your knock sensor doesn’t mean that there’s anything wrong with the sensor, but there could be. Another possibility is an engine knock.

Either way, you’ll get a check engine light, so you’ll have to narrow down the problem a bit further after you read the engine code.

2. Start engine

Although your engine control module should work fine with or without the knock sensor in most cases it could cause engine knocking and pinging in very rare instances.

Engine knocking and pinging can lead to engine damage. It is a fatal condition that could cause irreparable harm within seconds.

Find out more: Six Causes of Knocking and Pinging in Your Car Engine

3. Reduced Acceleration

As soon as your knock sensor gives you, you’re going to notice a decrease in acceleration. To prevent engine damage, your engine may default to lower outputs.

It reduces the risk of more damage and also lowers emissions, which allows it to be compliant with EPA standards.

4. Engine Power is lost

If your vehicle has a faulty knock sensor, it won’t only affect how quickly it can accelerate, but it’ll also lose its top speed and torque. That’s because those default values that your engine reverts to reduces your engine’s performance to keep your engine from producing too much and force and causing further damage.

But by reducing the amount of force, your engine’s computer also reduces the amount of power that the engine can produce.

5. Energy Economy is at Risk

Anytime that your engine defaults to default values, you’re not getting optimized performance. It can affect acceleration and power, as well as fuel economy.

This fuel economy reduction isn’t likely to be super noticeable, but you’ll probably see a drop by a few miles per gallon.

Knock Sensor Function

The knock sensor's purpose is to detect abnormal sounds coming from your combustion chamber. These unusual sounds are called knocks. They indicate an even deeper issue in your system and should be addressed immediately.

It will illuminate the check engine lamp and decrease engine performance if it detects a problem. This is an excellent feature that protects your engine in the event of an emergency. However, it can be frustrating trying to fix a knock sensor problem.

You’ll notice the exact same symptoms if you have a faulty knock sensor that you would detect if you actually had a knock in your engine!

That’s why you need to rule out a potential engine knock before determining that you have a faulty knock sensor. Otherwise, you might be replacing a sensor that’s doing exactly what it’s supposed to!

Keep in mind that not all knocks are loud, so if you don’t know what you’re looking for, it’s best to have a certified mechanic take a look at it and make a diagnosis.

Knock Sensor Locator

The knock sensor can be located in a variety of places, including the engine block and cylinder heads, as well as the intake manifold. Most commonly, it's bolted to an engine block. Keep in mind that your knock sensor needs to hear and feel what’s going on.

Because of the wide variance on a knock sensor’s locations, it’s best to have an idea of what you’re looking for before you start. A good way to do this is to look up your vehicle’s specific knock sensor on a parts site before trying to track yours down.

Finally, keep in mind that these are electrical connectors, so you’ll be able to tell if you’re looking in the right area if there’s an electrical connector sticking out of the back of the sensor. However, there are tons of different sensors, so make sure you’re disconnecting the right one.

If you’re not sure that you’ve found the right sensor consider taking your vehicle to a certified mechanic for repairs, so you don’t accidentally cause further damage.

Knock Sensor Cost Replacement

Depending on the vehicle you have and which repair shop it is taken to, an average cost for replacing a knock sensor will be between $250-$350. This is only if the knock sensor has failed and not if an engine knock is present.

Before you replace any components, make sure that you have a proper diagnosis that way, you’re not throwing money down the drain replacing the wrong parts.

If you’re completing the repairs yourself, you can save a few bucks by completing the repairs yourself. An aftermarket knock sensor costs anywhere from $20 to $100, depending on what you drive. Keep in mind, however that OEM parts are more expensive and OEM parts may be required if the vehicle is still covered by warranty.

If the repair is to be done by a mechanic, then standard labor cost for replacing a knock sensor ranges from $150 to $250. 

That’s because depending on what you drive, the knock sensor can be challenging to access without removing other components. You can expect to pay more for more parts that your mechanic must remove.

It is more likely, however that you'll need to hire certified mechanics. You can usually save hundreds of dollars if the work is done yourself.