The speed will not decrease once the treadmill is started. The speed will increase when the button is selected but will not decrease when selected. Everything else still works except the (speed down) button.

Please note that these instructions don't apply to all models of treadmills. Many recent models, including all models with touchscreen consoles, do not have the capability for the user to calibrate the speed.
Speed calibration is a process that is used to correct imbalances in the machine's operating speed.
Please note that this procedure involves pushing the machine to its maximum speed very quickly. Do NOT stand on the treadmill while performing this procedure.
First, you need to enter calibration mode on the console. To do this, press and hold the STOP and SPEED INCREASE buttons (this will be either an up arrow or a + (plus) symbol). While holding these buttons, insert the safety key onto the console. This will cause the machine to go into calibration mode. After this has occurred, you may let go of the buttons and the safety key.
Next, press the stop button once. This will enter advance the screen to the next feature of calibration mode. As said previously, do not stand on the treadmill while performing this procedure. Press and hold the speed increase button. Watch the console. You will see a number on the display that will increase as you hold the speed increase button. Continue to hold the speed increase button until the number changes to "85." This will take the treadmill to its maximum speed, which will also be displayed on the console. Please note that the example below only demonstrates the console displaying "85", not the treadmill's operating speed.

There would be some problem in wiring harness between motor control board and control panel. Try to remove form both side an reconnect it
If you wish to get some details; check the site linked here. Pull up older posts.
http://electro-medical.blogspot.com/

The micro switch is probably bad, the button (micro switch) needs to be replaced.

Treadmill console needs to be replace

book an engineer, sound like motor controller, loose belt could also be a issue, needs a safety check where are you based ?

Must enter the engineer mode. Turn the machine ON. Wait until you see the "Select user" screen.
1 - Press and hold the key to increase the incline and the key to decrease the speed for 3 to 5 seconds.
2 - Use the arrows to increase speed to get to Eng3 program (or the arrows next to the Enter button).
3 - Press "Enter"
4 - Then press the green start button until KM appears on the screen.

Remove the console from the treadmill and remove the back
remove and replace the ribbon cables that run from the keypads to the control board (they are flat plastic strips that push into the board).
remove and replace all the connections on the console
reattach the console and calibrate by doing the following:
1. With the key off press the speed + and stop buttons at the same time and hold
2. place the key on and release the buttons
3. press the stop key once
4. press the speed up til it reaches 85
5. if your max speed is 10 mph then it should read about 9.8-10.2

If the keys will not work then the console is bad and would need replaced.

Enter the testing mode after switching the treadmill POWER ON by pushing first ‘SELECT’ and then ‘SPEED DOWN’ and hold buttons pressed simultaneously.
LED scanning mode for verifying display functionality Press ‘ENTER’ to scan DATA LINES to verify display functionality Press ‘ENTER’ to scan SACN LINES to verify display functionality Press ‘ENTER’ to scan LED’s to verify display functionality After pressing ‘ENTER’ the TIME window shows “test” and the keypad functionality can be tested (the value displayed changes when a button is being pressed) Press ‘ENTER’ to enter IO mode (TIME display shows lift motor potentiometer value, SPEED reads speed from speed sensor and DISTANCE shows heart rate pulse) Press ‘START’ to test lower board relay (a “click” sound) and then press ‘ELEVATION UP’ for 2 seconds to increase the elevation to 8%. Press ‘ELEVATION DOWN’ to decrease the elevation to minimum percentage 0% Press ‘SPEED UP’ to increase speed value and ‘SPEED DOWN’ toto decrease speed value. Press ‘ENTER’ to repeat above test or ‘SELECT’ and ‘ SPEED DOWN’ buttons simultaneously to return to the normal mode
The lift motor calibration is done manually by following the following steps: 1. Drive the lift motor to 0% from user interface (inclination display must be 0%) 2. Switch off the treadmill and unplug the power cable 3. Remove motor cover 4. Fold up the running deck 5. Loosen two screws (A) attaching the lift motor nut (B) to the incline frame 6. Adjust the lift motor nut (B) so that there is 9 mm (.358 in ) gap between the upper end of the nut and the lift motor frame Tighten the screws (A) and verify correct elevation by measuring Adjusting lift motor to 0% position 2.76in 12% position 9.646 in measurement taken from floor to bottom of motor compartment frame.

Testing Mode 1. Enter the testing mode after switching the treadmill POWER ON by pushing first ‘SELECT’ and then‘SPEED DOWN’ and hold buttons pressed simultaneously 2. LED scanning mode for verifying display functionality 3. Press ‘ENTER’ to scan DATA LINES to verify display functionality 4. Press ‘ENTER’ to scan SACN LINES to verify display functionality 5. Press ‘ENTER’ to scan LED’s to verify display functionality 6. After pressing ‘ENTER’ the TIME window shows “test” and the keypad functionality can be tested (the value displayed changes when a button is being pressed) 7. Press ‘ENTER’ to enter IO mode (TIME display shows lift motor potentiometer value, SPEED reads speed from speed sensor and DISTANCE shows heart rate pulse) Press ‘START’ to test lower board relay (a “click” sound) and then press ‘ELEVATION UP’ for 2 seconds to increase the elevation to 8%. Press ‘ELEVATION DOWN’ to decrease the elevation to minimum percentage 0% Press ‘SPEED UP’ to increase speed value and ‘SPEED DOWN’ to decrease speed value. Press ‘ENTER’ to repeat above test or ‘SELECT’ and ‘ SPEED DOWN’ buttons simultaneously to return to the normal mode

To select the settings mode, first remove the key from the console. Then, hold down the Stop button while reinserting the key into the console.When the settings mode is selected, the display will show the word ENGLISH or METRIC to indicate which system of measurement is selected. To change the system of measurement, first highlight UNITS and press the User/Enter button. Next, press the increase or decrease button beside the User/Enter button to select the desired system of measurement. Then, press the User/Enter button.To exit the settings mode at any time, remove the key from the console.