There are four interconnect cables from the tree control board (located where the power connects to the tree) connecting to each of the light sets (front left, front right, rear left and rear right). A cable might have a loose connection at one end or the other. The tree control panel can be removed (3 screws - front bottom, back bottom, side bottom) and lowered out of the tree.
- Side screw location
- Front screw location (same on rear of tree)
The metal panel slides out the bottom of the tree exposing the tree control board and the four connectors. Determine which cable is connected to the failing lights by unplugging one connector at a time and powering on the tree. When the failing panel does not light up, that's the cable. You may be able to reach up into the tree to get access to the other end without having to disassemble the tree.