Water backs up into the sink when the garbage disposal is not running.

Safety first: Do not run the disposal while the sink is backed up.

• Use a plunger: Block the other sink drain (if present) and use a sink plunger over the disposal side to attempt to clear minor clogs.
• Clean the trap: Place a bucket under the P-trap, remove it, and clear any debris. Inspect the trap and cross tube for buildup.
• Snake the line: If the blockage is beyond the trap, use a drain snake through the wall stub-out to clear the line.
• Prevent future clogs: Avoid pouring grease down the disposal and always run plenty of water when grinding food.