What is a turntable coupler?

The glass plate sits on a wheeled roller ring and engages a small splined coupler driven by the motor below. Strip the coupler splines or wear the ring and the plate stops turning, leaving cold spots in the food. Both parts lift straight out of the cavity, cost a few dollars, and are genuinely owner-replaceable — and often the whole problem is simply debris jammed under the ring, which costs nothing but a clean.