Water backs up into the opposite sink bowl when the disposal is running (on a double-bowl sink).

Safety first: Turn OFF power to the disposal. Do not use chemical drain cleaners.

• Clean cross tube: Remove the horizontal tube connecting the two sink bowls and clear any debris or grease buildup.
• Inspect trap layout: Ensure there is only one P-trap serving both bowls, not two traps in series, per plumbing code.
• Clear common drain: Clean the P-trap and horizontal arm to the wall as needed.
• Test flow: Reassemble piping, then run water and operate the American Standard disposal. If backup persists, a deeper drain or venting issue may require a plumber.