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How to Tell if You Have a Bad Knock Sensor

By Mia Bevacqua, May 22, 2018

Your car’s knock sensor is a small part that can save you from a big repair bill. If it goes bad, you’ll want to get it replaced quickly. Let's look at what the knock sensor does, or you can skip ahead to the symptoms of a bad one.

A knock sensor is used to detect engine knocking or pinging, which results from abnormal combustion in the engine cylinders. If left unchecked, this bad combustion could ruin parts of your engine. Engine knock can also be cause by bad fuel, so if you hear it, try getting better gas next time.

 

When spark knock is present, the knock sensor sends a signal to the vehicle’s computer. The computer then retards — or delays — the ignition timing to reduce knock and prevent engine damage. 

Typically, the knock sensor is mounted on the engine block or cylinder head. Some engines have more than one sensor.

Get it diagnosed by a professional

Symptoms of a failing knock sensor

You might have a faulty knock sensor if:

  • You hear engine knock or pinging: The knock sensor should detect engine knock or pinging if it’s functioning properly. If it’s not, the car’s computer will not know to change the ignition timing, and you may hear pinging from your engine.
  • Your check engine light is on: The knock sensor sends a signal to the car’s computer when the engine is running. If the computer notices the signal is missing or is out of range, it will turn on the check engine light and set a diagnostic trouble code. (You can buy a scanner to check OBD codes in your car.)
  • Your car lacks acceleration power: Because the car’s computer uses knock sensor input to adjust engine timing, a faulty knock sensor can result in a loss of power. It’s typically overly retarded ignition timing that causes this lack of power.
  • You start noticing poor fuel economy: Ignition timing that is overly retarded can affect engine performance and worsen your fuel economy.

» LEARN MORE: Get an estimate for your knock sensor replacement

How to fix a bad knock sensor

The only way to fix a bad knock sensor is to replace it. You or your mechanic should test the sensor before declaring that it’s the problem. This is done by attaching a gauge, such as an oscilloscope, to the wire that runs from the knock sensor to the car’s computer. When it’s working, the sensor produces a steady voltage — simulating engine knock by tapping lightly on or near the sensor should cause its output to fluctuate. 

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