I have a single basin sink with a garbage disposal. We never put "difficult" things down the disposal. However, twice we have had a problem in the past year. Water will drain fine when the disposal is on. If the disposal is off, the sink will fill up immediately. I have tried drano which has had no affects. Any ideas?

It sounds like you have a partial blockage you need to snake it see the image below, good luck.

John

John's "bang on!" You do have a partial blockage, REmove the "J" bend from the trap and send a snake up into the wall, (see image) You will hit a bend about 8 inches in but once past that you only have 6 more feet to put out, Flush with a pan of boiling water,

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Water will drain fine when the disposal is on. If the disposal is off, the sink will fill up immediately.

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There's a reason for that. The disposals table is acting like a centrifugal pump. If you would have had an double sink it would have pumped from one side to the other. Mystery solved? Good luck, Tom