Your kitchen sink has several possible drain line configurations depending upon what year it was built, whether it has a garbage disposal and/or a dishwasher.
To properly diagnose a kitchen sink stoppage, you are going to have to discover what type of drain system is installed. Look under the sink and see how many drain lines connect from under the sink to the wall. You should see one trap for one drain line or 2 traps for 2 drain lines.
If you have one trap, then you also have a 2 part waste. This series of pipes combines 2 sink drains together into one trap using a tee fitting. If you have a garbage disposal in one of the sinks drain openings, then the tee fitting will be a baffle tee. The baffle inside the tee is what keeps the water from the discharge of the garbage disposal from coming back up and out of the drain in the other sink compartment.
If you have a kitchen sink back up and standing water is just in the garbage disposal side of the sink while the other sink basin flows fine, then you have an obstruction in the baffle tee. The tee will need to be removed, cleaned and reinstalled in the drain system. If you have standing water in both sinks, then you have a stoppage anywhere starting at the bottom of the baffle tee up to a point to where the kitchen drain line branches into the main house sewer line.
To pinpoint more precisely where the stoppage is located, fill both sides of the sink with water so that you can see the water at the opening of the sink drains. Then carefully remove the cap or plug from the outside kitchen sink cleanout plug to see if the drain water is filling the line. If water pours out of the cap or plug, then the stoppage is somewhere between the cleanout plug and the connection where the kitchen sink line connects to the main house sewer. In this scenario, the blockage can be cleared from the kitchen sink cleanout access. However, if no water comes out after removing the cap, then you will need to get a bucket and a pair of large adjustable pliers and remove the trap from underneath the sink.
You will have to take your time and remember that the water from both sink compartments are going to have to fit in that bucket so make sure you have enough containers standing by to handle it. Rinse the trap outside with the garden hose and before replacing it, run a little water in both sinks and let the water drain into the bucket. Now, test the drains by refilling them. If the water continues to flow you’ve fixed it, the stoppage was in the trap. If the basins clog back up, then you will have to have the kitchen sink line snaked with a drain cleaning cable. Call your local plumber to clear the drain.
If you have the two separate drain lines with 2 different traps, fill both sides and see if they are holding water. As explained earlier, to pinpoint more precisely where the stoppage is located, fill both sides of the sink with water so that you can see the water at the opening of the sink drains. Then, carefully remove the cap or plug from the outside kitchen sink cleanout plug to see if the drain water is filling up inside the line. If water pours out of the opened cap or plug, then the stoppage is somewhere between the cleanout plug and the connection where the kitchen sink line connects to the main house sewer. In this scenario, the blockage can be cleared from the kitchen sink cleanout access using a drain machine cable or hose bladder.
If no water comes out of the opening, and both sinks are holding water, chances are you don’t have two simultaneous trap blockages so the higher trap will have to be removed and a snake cable will most likely be needed to clear the blockage. If however, just one of the sinks is stopped up with water and the other flows freely, there is a very good chance that the blockage is inside of the trap of the clogged sink. Remove the trap, rinse with a garden hose outside and reassemble.