Thank you brother.
That causes can be
1) optical infrared speed sensor is obstructed.
2) drive motor brusher are bad.
3) speed sensor position is incorrect
4) speed sensor potentiometer adjustment is incorrect
5) speed sensor is faulty.
6) lower PCA is bad
7) lower PCA to upper PCA interconnect cable is bad
8) upper PCA is bad
Remedies: vacuum the drive motor pay attention to speed sensor .
b) motor brusher do not make suffciente contact repair
c) adjust the speed sensor see the scheme attached.
d) locate the speed sensor potentiomente on the lower PCA.set the potentiometer in mid-range
e)measure voltage D.C. for 5vdc, two fo the leads will toggle between 0vdc-5 Vdc.as you rote the speed sensor.
f) substitute a known good lower PCS for determine g) substitute a known good interconnect cable h) substitute a known good upper PCA for determine if is faulty.
God bless you
see the block diagram and my personal TIPS for motor in treadmills