# How does a knock sensor work

**URL:** <https://boards.straightdope.com/t/how-does-a-knock-sensor-work/671260>\
**Category:** Factual Questions\
**Created:** [October 14, 2013, 10:31pm UTC](https://boards.straightdope.com/t/how-does-a-knock-sensor-work/671260 "2013-10-14T22:31:56Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\
**Post date:** [October 17, 2013, 4:49pm UTC](https://boards.straightdope.com/t/how-does-a-knock-sensor-work/671260/21 "2013-10-17T16:49:22Z")

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> [@Martian Bigfoot](#):
>
> -Knock, knock.  
> -Who’s there?  
> -Sensor.  
> -Sensor who?  
> -Sensor not doing anything, ya mind keeping an eye on the ignition timing?

-Knock, knock.  
-Who’s there?  
-The one who knocks.  
-‘The one who knocks’ who?  
-Ping ping ping ping ping…

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**Author:** ![Vuaren](https://avatars.discourse-cdn.com/v4/letter/v/d78d45/32.png) [@Vuaren](https://boards.straightdope.com/u/Vuaren)\
**Post date:** [February 18, 2016, 5:17pm UTC](https://boards.straightdope.com/t/how-does-a-knock-sensor-work/671260/22 "2016-02-18T17:17:58Z")

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I have a question on how the knock sensor is powered and how it communicates to vehicle. I have a 2007 Honda Odyssey and recently replaced the knock sensor. After I located the old sensor, I found some creature had chewed away the wire that connected to it.

Th wire harness that plugs into the knock sensor contains on a single wire. How does that work? Is that wire power? Positive or Negative and how does the same wire talk back to the car’s computer?

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**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\
**Post date:** [February 18, 2016, 6:08pm UTC](https://boards.straightdope.com/t/how-does-a-knock-sensor-work/671260/23 "2016-02-18T18:08:01Z")

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> [@Vuaren](#):
>
> I have a question on how the knock sensor is powered and how it communicates to vehicle. I have a 2007 Honda Odyssey and recently replaced the knock sensor. After I located the old sensor, I found some creature had chewed away the wire that connected to it.
> 
> Th wire harness that plugs into the knock sensor contains on a single wire. How does that work? Is that wire power? Positive or Negative and how does the same wire talk back to the car’s computer?

Only some microphones (which most knock sensors are) require an input voltage (condensor types). A piezo sensor, which **zwede** [mentioned above](http://boards.straightdope.com/sdmb/showpost.php?p=16757116&postcount=3), only needs an output wire, since the piezoelectric effect directly generates an electrical current from the input mechanical force (sound). The “other wire” needed for a complete electrical circuit is ground, and in a car it’s almost always the case of the device itself attached to the ground circuit which is the frame of the car (and everything bolted to it).

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