A sink trap, or P-trap, is the curved part of the plumbing underneath a sink. It is there to keep sewer gas that bypasses from the vent from coming in to the bathroom. The traps are prone to clogs, and will need to be replaced periodically. Replacing this is an easy do-it-yourself job that will take less than 20 minutes and usually costs under $20.
Step 1 - Prepare the Area
As you remove the old trap, water and other gunk may still be present. Set up a small bucket directly underneath the trap to catch any of this. Having some old rags handy will also allow you to keep the area under the sink dry while you work.
Step 2 - Remove Old Trap and Purchase the New Trap
It's a good idea to remove the old trap and take it with you to purchase the new one. This will eliminate the need to cut any pipe later, and will ensure you've chosen the correct size. To remove the old trap, simply unscrew the nut that secures it to the wall pipe, and then remove the nut that secures the trap to the drain tail piece. You can loosen the nut found in the middle to allow you to manipulate the trap better.
Step 3 - Check to See if Replacement is Necessary
It's not uncommon for people to assume the trap needs to be replaced, just to find out that it only needs a thorough cleaning. If there is a lot of build up in the old trap, you can clean it out with a wire brush and reinstall it. You will still need to follow the steps to replace the gaskets.
Step 4 - Fit the New Trap
Test fit the new trap before you install it. This is to ensure everything fits and the distance is correct. If the tail piece is too short, you will need to use a slip joint extension. This will attach to the tail piece the same way the trap attaches.
Step 5 - Gaskets
Place the gaskets over the tail piece. Then slide the nut in to place.
Step 6 - Put Together New Trap
If you are working with PVC pipe, a gasket won't be necessary for this part. For exposed sinks, chrome will require a gasket, and the trap should have come with one. Use pipe thread compound over the threads to ensure a tight seal and prevent leaks. Make sure the nut on the trap itself is snug, but don't completely tighten it yet.
Step 7 - Install the Trap
Now that the trap is put together, you can begin to install it. Use the thread compound again where the trap will come in to contact with the tail piece and the wall pipe. Hand-tighten the nuts over the gaskets. Do this for each end. Once the trap is in place and secure, you can go back and tighten the nuts with the wrench.