The lift position actuator does not physically operate the zero switch.
Bad lower PCA.
Bad zero switch.
Inspect for the presence of a magnet in the magnet holder. Replace magnet if missing. Check for output of the rotation sensor and follow signal all the way to the upper board. If the sensor has no output, replace it. If the signal drops out at the cable, replace the cable.
The lift platform mounting or the zero switch mounting plate could have physical problems that could cause the actuator knob to miss the zero switch as it moves past. Inspect and repair the lift platform mounting if necessary. Inspect and repair or replace the lift switch mounting plate if necessary.
5VDC is provided to the zero switch from the lower PCA. You should get 5VDC across the zero switch when it is activated. If the reading is considerably low, remove both wires from the zero switch and retest for voltage directly through the wires. If the reading is still considerably low, investigate the zero switch wiring. If the wiring is good and connected properly, replace the lower PCA.
When a zero switch is bad or not actuated, the system will go through the entire search process and continue past the home switch to find the lower limit switch. Move the lift to a position where the zero switch is not activated and remove power from the treadmill. Remove both wires from the zero switch and check the zero switch with an ohmmeter. The zero switch should read near 0 ohms. While the ohmmeter is still connected to the zero switch, activate the switch by hand and the reading should change to infinite (open). If either reading is considerably incorrect, replace the zero switch.