Plastic drainpipes under you sink may leak from a number of locations and for a number of reasons, but you can repair or correct these leaks yourself. Plastic pipes do not require the same level of expertise to repair that metal pipes require, and the pipes are cheaper to replace should you opt to do so.
Locate the Leak
You must locate the leak under the sink before you can do anything about fixing it. Sometimes you may see an obvious leak if one of the plastic pipes has a large crack or hole, but many times the leak proves difficult to locate. The leaking water will often run along the pipes before it finally drips to the bottom of the cabinet under the sink, making finding the leak a difficult task. You must dry off the pipes while you stop up the sink's drain and fill the sink with water. Once the sink is full, remove the stopper and shine a flashlight on the pipes. You should see water begin to bubble out of the leak in the pipes.
Look at the pipes under your sink to see if any sit crookedly, which will lead to leaks. A crooked pipe is a loose pipe and will leak water until you straighten the pipe and tighten its connections. Before you can straighten a crooked pipe, you must first loosen the connections on either end. Force the pipe so it sits straight, and then tighten the connections by hand as much as possible. If the leaking continues, use a pair of adjustable pliers to lightly tighten the connection without marring the plastic.
Home improvement and plumbing supply stores sell repair kits for plastic drain pipes, which consist of pieces of plastic that fit around the leaking section of pipe and glue to hold the patch in place. Other kits use rubber washers that sit over the breaks and metal bands that squeeze the washers in place. You can also apply epoxy that is labelled for use with plumbing, which is malleable and will seal off leaks in pipes, hardening after it dries completely. Using epoxy that is not for plumbing will not work, since other epoxies cannot be exposed to water before they cure.
Replace the Pipe
You may opt to completely replace a leaking pipe rather than fix the problem. When you install the new pipe, trim it with a hacksaw so it fits in the space provided; otherwise pipes will bend and a new leak will start. Tighten the connections on both ends of the new pipe using your hands. If the connections leak after, you can tighten them gently with a wrench.