My kitchen sink and washing machine are tied into the same line. The sink drains fine, but the washer has started overflowing out of the waste pipe and gurgling up in the wash basin. Because the washer is in the basement, the prior owners installed an inline vent. Could this be malfunctioning?

No, you have a blockage or partial blockage in the drain between the washer and the main drain. Snake the washer drain to clear blockage.

AAVs can fail,you have one but it is not your problem. Harold gave you the right advice. Also you, if your washer goes into a stand pipe it should be 2" and at least 36" tall

AAVs can fail,you have one but it is not your problem. Harold gave you the right advice. Also you, if your washer goes into a stand pipe it should be 2" and at least 36" tall

Thank you both. The stand pipe is the correct specifications, so I guess I've got a blockage...
Thanks again

Washing machine connected to Kitchen sink drain is always a problem. Kitchen sink is the dirtiest waste pipe in your house. Washing machine needs clean, unobstructed 2" waste pipe to absorb rapidly-discharging washing machine. If you can, separate these two sewer lines.

Milo gave it to you straight. Grease and garbage from the sink mixed with grease and fiber from the washer is a blockage waiting to happen. And yes! AAV's and any other mechanical part can fail. Cheers, Tom