Calibrate bowden length not working?

I cannot seem to calibrate the bowden length in the service menu of the MMU2. I can get to the menu, and to the calibrate section(selection 4), but when trying to start the calibration process it just blinks a single red LED on the 1 filament spot.

Start by reproducing the service-menu flow through MMU2 calibration, specifically selection 4, and record the LED behavior when calibration starts. Trace the calibration entry point and determine why the process stops at a single red LED; done means bowden-length calibration starts and completes normally.

Written by the indexing model from the issue text.