Dacor Downdraft Ventilation Works Only on High

I have a Dacor ERV30 raised ventilation unit and REMP16 blower system for my Dacor Millenia 30" range. The unit raises and lowers normally, but the infinitely variable switch on the ERV30 now works only in the highest position. All units are about two years old. Any ideas?

I’m considering ordering the potentiometer harness kit, part# AP39563, but I’m unsure if that’s what I truly need.

I’m also unsure how to open the unit. Once raised, the back panel appears to be attached with rivets. So I wonder how to access the innards.

Thanks for any help that may be provided.

Warren