Why does the downdraft vent raise and lower continuously?

Problems with the vent limit switches or the up/down activation switch can cause the downdraft vent to move up and down continuously.

Limit switches detect when the vent reaches the upper and lower travel limits. If one of the limit switches breaks, the vent will raise and lower continuously. Disconnect electrical power to the downdraft vent and test continuity through the limit switches using a multimeter. Replace a limit switch if you measure the same continuity through a limit switch when the switch is activated and deactivated.

If the limit switches are okay, check the position of the up/down activation switch with power to the downdraft blower still disconnected. The vent must be in the raised position to check the activation switch, so lift the vent if it's in the down position and tilt it slightly so it stays raised. Remove the switch cover from the right end of the vent. Check the position of the up/down activation switch plunger. If the up/down button pad constantly presses down the plunger, loosen the up/down activation switch mounting screws and move the switch down until the bottom of the button pad membrane barely touches the plunger. Tighten the up/down activation switch mounting screws and replace the switch cover.

If the up/down activation switch position is okay, check continuity through the switch using a multimeter with electrical power disconnected from the downdraft vent. If the meter measures near 0 ohms of resistance with the switch plunger released, replace the up/down activation switch because it's letting current flow to the vent gear motor constantly, making the vent constantly move up and down. The up/down activation switch should stop current flow to the vent gear motor when the up/down activation switch plunger is released.