It took me 5 hours just to remove the knock sensor in my 1999 Honda Accord EX 4-cylinders. Since I have learned the lesson the hard way without removing intake manifold, I'd like to share with the community the easier way to remove/install it.
The knock sensor is located underneath the intake manifold and above the metal coolant hose, so you have little space to get it out. I ended up bought three different sizes of sockets to get it out. My pain is your gain.

Knock sensor is 24mm socket size, but since there is no 3/8" 24mm available, you have to use 1/2" 15/16 long socket. DO NOT use impact socket, it's too thick to get into the knock sensor.

1) You need to jack up the car on the driver side.
2) Remove the oil filter and the wiring harness close to the knock sensor. I ended up cutting off the cable tie of the wiring harness just to give myself some space to work on it.
3) If the knock sensor is broken (my Accord's knock sensor connector came right off when I unplugged the connector from it), just use the 15/16" socket and extension to un-screw it.
4) Do the same thing in reverse when you install it back. Don't forget to re-install the oil filter and check all the wirings.

Hope this help someone who will do the same job as mine.

 

And don't buy cheap ebay made in China knock sensors - I did and it did not work. Soldered and glued the old original one - no probs since then. Price of new OEM unit is quite stunning.

 

When it comes to critical parts especially electronics, I'd rather pay extra dollars for brands that I can trust. I ordered most of my components from Rockauto.