Where is knock sensor located on 2003 Chevrolet 2500HD ?

On Chevy 2500HD the knock sensors are located under the intake manifold.The Intake manifold has to be removed to Access knock sensor.

Remove the intake manifold.
Gently pry up the rubber covers.
Disconnect the knock sensor electrical connectors.
Remove Knock senor.

While installing Knock sensor.Tighten the sensor to 20 N.m (15 lb ft).

This will help.

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For more car related problem troubleshooting:---

This are the sections, you can go through and click the link to read its troubleshooting.

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The car will not start?

http://schematicsdiagram.blogspot.com/2011/06/car-will-not-start.html

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Car will not crank?

http://schematicsdiagram.blogspot.com/2011/06/car-will-not-crank.html

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Car has no spark?

http://schematicsdiagram.blogspot.com/2011/06/car-has-no-spark.html

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How to troubleshoot car battery?

http://schematicsdiagram.blogspot.com/2011/06/how-to-troubleshoot-car-battery.html

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How to test starter solenoid?

http://schematicsdiagram.blogspot.com/2011/06/how-to-test-starter-solenoid.html

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How to test cars starter?

http://schematicsdiagram.blogspot.com/2011/06/how-to-test-cars-starter.html

How to replace starter in car?

http://schematicsdiagram.blogspot.com/2011/06/how-to-replace-starter-in-car.html