When unit retracts, motor does not shut off and continues to hum. I must unplug it to get it to stop. It is fully retracted when this happens. It will not raise either.

My problem is identical to what you described.
Used GE instructions I found inside the electrical box to dismantle unit.
Discovered the problem was the up/down motor. Motor was making noise like it was spinning. Just did not have enough juice to fully turn over and raise the vent. If you gave it a little assist it would work. I have ordered a new motor. It's not too bad to get at once you get it apart.

My GE Profile downdraft system has a problem that seems related to the other posts here. The on-off switch (it's the 30" model so the switch is incorporated into the fan housing) works to make the system go up and it will turn on so the fan works. I can turn the fan off (using the separate switch for that), but the on-off switch in the housing doesn't work to allow the system to lower itself. I know the electricity to the device is working (the fan works) but maybe there is a problem in the switch itself. Sometimes it makes a noise, when I push the button, but I can't find out what to do to get it to lower itself.

I did not at any time feed it ************, so that's not the problem.

Any advice is appreciated. Perhaps it's the switch, itself (although it seems to work to make the device go erect).

Thanks,
Mark Uhlfelder