If your sink is slow to drain or stops draining completely, there’s probably a blockage somewhere in your waste pipe. A common cause is grease in the trap. If the water doesn’t drain at all, then you’ve got a complete blockage, which means there’s an obstruction in the waste pipe. There are a number of ways you can sort this out:
If your sink is completely blocked, partially fill it with water and place the rubber cup of a sink plunger over the plug-hole. Stuff a damp cloth into the overflow pipe outside to prevent any loss of pressure. Then pump the plunger up and down vigorously. Take the plunger away and see if the water will drain. You may need to repeat this a few times.
If that fails, try using a chemical drain-cleaning product. HG Duo Unblocker works well but remember to wear protective gloves and goggles when you use it, and make sure you follow the instructions carefully as this cleaner is toxic. Don’t use it around other chemicals such as bleach, as doing so could release dangerous gases. It’s also a good idea to spread some petroleum jelly around the plug-hole to protect it from damage.
If the blockage still hasn’t cleared, you’ll have to remove the waste trap under the sink. Put a bucket under it to catch any spillage, then unscrew the trap and empty the contents into the bucket. If you don’t find a blockage in the waste trap, use a drain auger to probe further into the waste pipe, pushing it down through the pipe towards the overflow outside. Once you’ve cleared the problem, clean the trap and put it back together, making sure you replace any washers or O-rings. But don’t over-tighten it as it maybe difficult to undo later on.