If there is a leak underneath the bathroom sink, there is probably a problem with the supply connector. The connector is the piece of tubing that connects the faucet to the hot and cold water supplies. A number of factors can cause the leak; this must be determined early on to resolve the issue. Below is a guide to work it out yourself.
Step 1 - Inspect the Leaks
Turn off the faucet but not the water supply. Underneath the sink, check where the leak comes from. It can be directly under the faucet or the handles. If the leak does not come from any of these locations, it can come from the connection between the connector and the water supply lines.
Step 2 - Turn off the Water Supply
Turn off the water shut off valve located underneath the bathroom sink or beside it. Never work on the connections below the sink if the water supply is still on. If the shut off valve happens to be stuck, turn off the water supply for the entire home.
Step 3 - Repairing the Leak
If the leak occurs where the water supply lines are connected, try tightening the connections using a basin wrench. Turn on the water supply and check if the leak is fixed. If it is not, turn the water supply off again and remove the water supply lines. Wipe the male threads on the connector with a dry cloth and apply 2 to 3 wraps of Teflon tape around them.
Reconnect the water supply lines, turn on the water, and check for leaks. The water supply lines need to be replaced if they still leak after doing the repairs. If the leak comes from the connection between the connector and the faucet or the handles, make sure that the connections are tightened. If this does not solve the problem, remove the entire connector underneath the faucet, apply Teflon tape on the male threads of the faucet and handles, and reconnect it. Turn the water on to check if the problem is solved. If not, it is necessary to install a replacement. This is necessary especially if the threads are corroded or loose.
Step 4 - Replace the Damaged Connector
Purchase a new supply connector from a plumbing store. Detach the water supplies from the old one and loosen the connection between it and the faucet. Pull it out and set it aside. Unpack the replacement and inspect it for damages. Connect the middle portion into the faucet’s male threads. Tighten the connection with a basin wrench. Connect the supply tubes to the male threads underneath the two faucet handles and tighten them securely. Connect the water supply lines (hot to the left and cold to the right), making sure to apply Teflon tape on the male threads to ensure watertightness. Turn on the water and test the installation for leaks.