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If the vent retracts immediately after you tell it to raise, it means that your limit switch which tells the vent when it's in the up position isn't triggering or the switch is bad. This is quite a bit of work especially depending on how the venting is run to the downdraft but you'll need to go below the cooktop and remove the entire front sheet metal panel of the downdraft so that you can see the lift motor. Behind that motor are 2 micro switches. Sometimes the lever arm attached to the motor gets loose and needs to be re-installed so it hits the switches properly, but if the arm isn't an issue, replace those micro switches.
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