Downstairs toilet backs up with water when flushed,air bubbles appear,also bathroom sink is making gurgling noise after using water

Sewer lines are connected to one another and no "p" traps installed on either connection that's your problem !

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Originally Posted by anniemt22

downstairs toilet backs up with water when flushed,air bubbles appear,also bathroom sink is making gurgling noise after using water

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Are you non city sewer or a septic tank?
The downstairs toilet has a blockage downstream from the toilet. This can be reached through the bathroom sink roof vent, Rent a sewer snake and run it down the bathroom sink roof vent, Put out enough snakem to reach the base and 18 feet more. Good luck, Tom,