PVC drain pipes can fracture over time, or if a heavy weight is placed upon them. If the crack/damage is small, it can be fixed with RTV silicone sealant. This sealant (RTV stands for Room Temperature Vulcanizing) comes in a tube that fits in a standard caulking gun. The sealant does not need to be mixed, but starts to cure when it comes out of the tube and is exposed to moisture in the air. Consequently, it needs to be applied quickly, before it sets.
Locate the damaged area on the PVC sewer pipe. Do not use the sewer line while fixing the leak.
Wipe around the leaking area of the pipe, making sure that all dirt is removed. Then dry the area with a towel.
Place the sealant tube in the caulk gun, and cut off the end of the tube's plastic nozzle with a utility knife.
Squirt an ample amount of sealant quickly over the crack, and smear it over the crack with a putty knife. Wait for the sealant to dry completely, before running waste through the sewer pipe.
Wipe off the sealant from the putty knife, before the sealant has time to dry. For larger cracks, it is recommended that the cracked area be cut out from the sewer pipe, and that a new piece of sewer pipe be installed.