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There are 2 knock sensors located below the upper intake manifold. With the intake removed follow wires to the middle of engine near the front and back. The sensors are recessed in the lower valley pan and covered by some type of insulating material. The rear sensor usually fails due to water working past the insulating material and filling the hole with water and covering the sensor, causing it to rust. Remember to replace the upper intake gaskets and injector o-rings. You dont want any other driveabiliy issues after this repair.
usually the sensors (2) are good but the harness goes bad and there is corrosion at the knock sensor connection. The intake manifold need to be removed and the harness replaced. KNOCK SENSOR 2 IS THE REAR.
I assume you are wanting to know how to replace the knock sensors?
This vehicle has two knock sensors and they are located under the intake in valley of engine.
you will need to remove intake to gain access. This will be done by disconnecting injector wiring various sensor wires and moving harness over. Remove fuel lines at injector fuel rail. loosen intake bolts and remove intake. there will be some hoses and air intake etc you will have to remove before removing intake. Once intake is off you will see two rubber plugs with wires coming out this is where knocke sensors are pull plugs up and squeeze the connector to release it from knock sensor.Most likely you will find one with moisture in it this will need to be cleaned out.Replace both sensors since you are there if sensor harness looks damaged or connectors corroded replace it as well.Gm recommends after rplacement clean around the plugs and make a U shaped dam around them with RTV sealer. this will make any water deflect around the holes. Being that this is an 04 replace the intake gaskets They say you can reuse them but I dont trust that unless it is a new vehicle.Thanks for choosing us let me know if i can assist you further with this or in the future.
I had to do this last summer. First I went to Advanced Auto. They read the code for me. One of them was bad, but I went ahead and replaced both. I purchased a manual for the steps, two knock sensors and a gasket set. About 170 dollars total. You will also need a torq wrench in inch pounds. Take your time, keep it clean. Make sure you clean the manifold and engine head surfaces where the gasket seats. Use a non metallic scraper, and if need be solvent to loosen any old gasket material. I found that laying saturated rags on the head surface and leaving them set a few hours helped the most. You can also get the steps and torque specs from either Advance auto or Auto Zone(then you don't need the manual). Use patience when tightening the manifold bolts, you should torque them in the sequence given(in the manual or online) and you have to do it twice, first time at half torque, then the second at full torque. If you over tighten you can crack the manifold. Good luck Art LeDoux, Stillman Valley Illinois