This is for the 1.8tl JH1 injected engine. The O ring surrounds the idel adjustment screw on the throttle body and servers 2 purposes. First it creates a seal around the screw and 2nd it helps hold the screw in place with friction.
When to replace:
Example of a 'flat' worn O ring that needs to be replaced.
Take note of the screw's position - how far it is screwed in (I took a picture with my phone). Then, simply unscrew the idle screw. It is about 2 inches long. If the O ring needs replacing it will either be hard, cracked or flattened. Screw is located here.
Remove the old O ring. You may need to cut it. Clean up the idle screw whilst it's out of the engine.
Push the new O ring in position. I used a little bit of oil to help.
Replace the screw returning it to it's old position. Turn on the car and adjust idle speed. Check with both electrics on and off.