Symptoms of a Bad Knock Sensor (and What it Does)

Last Updated on April 24, 2020

In this post we will discuss one part in the engine system known as “knock sensor”. After reading, you will know the basic functions of a knock sensor and common symptoms of a faulty knock sensor.

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What Does a Knock Sensor Do?

The main function of a vehicle’s knock sensor is to sense for vibrations that are caused by the detonation or knock of the engine. A knock sensor is located on the intake manifold, the cylinder head or the engine block.

Once vibrations are sensed, the powertrain control module receives a voltage signal and uses it to change the timing of the ignition and stop it from detonating. By doing this, the engine will be protected from damage by pre-ignition.

Remember that denotation in an engine is basically when fuel explodes in its cylinder and alters the timing of the ignition. So if the knock sensor fails, then this detonation cannot be regulated and may result in major damage to your engine.

That is why it is important to recognize what the symptoms are of a failing knock sensor. Then you can replace it and avoid paying thousands of dollars fixing a damaged engine.

Common Bad Knock Sensor Symptoms

what does a knock sensor do

When a knock sensor goes bad, the symptoms will usually be noticeable when you’re traveling at high speeds like on the interstate. The acceleration and fuel mileage will start to decrease for one thing.

Also, your car’s computer system should notify you when there is a problem with the knock sensor. Either that or you’ll see the “check engine” light flashing on your dashboard.

In rare circumstances, cheap or dirty gasoline will improperly cause the powertrain control module to falsely warn you that you have engine problems when you really don’t. So if you really want to this, then you can use super unleaded gasoline instead of regular unleaded or use a fuel additive.

Aside from warning lights, you can also listen for noises that indicate a failing knock sensor. These will be thumping sounds that will come directly from the engine.

As the knock sensor gets worse, the thumping sounds will gradually get louder and louder. This noise occurs because the air and fuel ignite in the cylinder rather than getting to the combustion point.

Other symptoms include an engine misfire when trying to start the engine or even vibrating and shaking too. You may even see strong exhaust fumes emitting from the engine which gives off a burning smell in the car. Again, this is from the denotation of air and fuel in the cylinder.

The worse symptoms will involve the car jerking, hesitating or dragging while in motion. Don’t let it get to this point. Listen for the early warning sounds and then take your car to an auto repair shop to get new knock sensors.



  1. Thank you for writing this post. I have a 1996 Subaru Legacy Outback with the 2.2 engine. It is running rough. Check engine light came on. It’s showing four codes. I am replacing 02 sensors, knock sensors, MAP sensor, and idle control sensor. After reading the symptoms of these sensors, I think my car will run better.

  2. I have a 2003 Honda Accord and the sensor went bad and I experienced most of the symptoms and right now my transmission is really acting up, could that be the cause as well?

  3. I have a 2001 s10 2.2l 5 speed, it has all the symptoms but no check engine light. Does a bad knock sensor always throw the light?

    • “Always” is a strong word; if I said it always throws a check engine light, I’m dead wrong as soon as you find a single counterexample. 🙂

      That said, a bad knock sensor should throw a check engine light, yes. What exactly are your symptoms? I am inclined to guess that the root cause of your issue is something other than the knock sensor itself.

  4. 2004 Honda CR-V, it has check light on car is hesitant, replaced Knock sensor as this was the code, checked the wiring from sensor to ecu and has continuity so wire seem ok, any other suggestions to check next? Thanks

    • What exactly was the code? It sounds like you’re on the right track, but I get the feeling that there’s more going on.

      By the way, a sensor can have continuity and still give a voltage that is outside acceptable parameters, throwing a check engine light.

  5. I have a 2004 Toyota vitz. I had knock sensor issues and got it replaced. Now, even though the check engine doesn’t appear anymore, I am still experiencing all the other symptoms like engine sound, loss of acceleration, bad fuel mileage. Any idea? Should I change the knock sensor again even if the check engine light is off? I am really confused, I will appreciate your feedback.

    • What unusual engine sounds are you experiencing? Check your fuel trims to make sure your air fuel ratios are about where they should be. If they are extremely lean or rich, you likely still have a mechanical problem that is causing the loss of acceleration.

      As a sanity check, you may want to test that your new knock sensor is functioning properly. You should be able to find a procedure for this in the factory service manual.


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