Where are the spark plugs located on a 1994 Nissan Altima?

They are located in the top of the engine block and can be accessed through the very top of the engine.

On top of the engine you will see four rubber things with wires leading off to the driver's side. These are the plug wires that connect the spark plugs to the distributor.

To get to the spark plugs you have to pull the plug wires out. Do this one at a time and be careful not to destroy the wireset by jerking them out. Grab one of the rubber pieces and pull up; applying pressure as you wiggle it. You may have to apply quite a bit of pressure but if you continue to move it back and forth it should let go of the plug.

Once you have taken a wire off, you can use a flashlight to see the plugs down the holes in the top of the engine. To remove the plugs you will need a "plug puller" which is a deep socket wrench with a material in the top of it that the top of the plug will stick into. This allows you to unscrew the plugs and then pull them out of the engine block. It also allows you to lower your new plugs into the engine. One can be purchased at any decent auto parts store. **Be careful when removing and replacing the plugs - if too much pressure is applied the glass parts of the plug can break and possibly drop debris into the cylinders.

**If you are inexperienced, label the plug wires to the cylinders (1234 left to right). This will ensure that you don't somehow place a wire on the wrong cylinder though the lengths are staggered so it shouldn't be a problem.