Based on the model number you provided I'm sure I can help you. We will start by checking the reed switch on the incline motor. If the incline motor runs for a few seconds we can get the incline to calibrate itself very easily.
Insert the safety key. If the incline motor starts to run let it go thru the entire calibration on it's own.
If the incline does not start on it's own, start the incline motor running then pull the safety key.
Reinsert the safety key and allow the incline motor to go thru the calibration on it's own.
If the incline motor will not start at all you will have to remove the motor hood and gain access to the incline motor.
To remove the motor hood follow the steps below.
Unplug the treadmill.
Fold the treadmill up to the storage position.
Lay the treadmill down with the walking board on the floor. This way you will gain access to the motor hood screws.
Remove the motor hood screws from the bottom of the treadmill.
Stand the treadmill back up and unfold it.
Remove the last motor hood screw and remove the motor hood.
On the side of the incline motor there are 2 small black wires. They are attached to a reed switch that is under the black rectangular cover. remove the reed switch and clean the excess grease off of it.
Reinstall the reed switch and make sure that the wires are lined up with the notch in the cover and that they come out without being pinched. The cover only fits one way.
Test the inline operation if it is working properly install the motor hood in the reverse order of steps 1 thru 6.
If this fails the problem may be the incline motor itself. The motor may be purchased at
Sears Parts Direct
I have added a link to get a copy of a Pro-Form Trainer 420 treadmill
I hope this helps.
If I can be of further assistance please feel free to submit any questions you have along with the specifics.
Thanks for using Manage My Life.