Garbage disposal turns on but water backs up into the second bowl of a double sink.

Safety first: Turn off power to the disposal before dismantling any plumbing.

• Check waste kit layout: Ensure the disposal outlet connects to the waste kit in accordance with Rangemaster and sink manufacturer diagrams, with adequate fall.
• Clean shared pipework: Remove and clean the P-trap and any T-pieces serving both bowls. Clear grease and food build-up.
• Adjust trap height: Ensure the trap is not higher than the disposal outlet, which can cause backflow into the other bowl.
• Test flow: Reassemble, run water in both bowls, and operate the disposal to confirm that water no longer backs up.