Treadmill won't slow down

What make/model is the treadmill? What speed does this behavior occur? Does the walking belt slip, while the motor and rollers continue to turn? If the walking belt is slipping then the motor belt needs to be tightened. If the walking belt slows with your weight on it, does this happen at 1-3 mph and still occur at 4-5 mph? It could be your body weight causing it to slowdown, try to increase the speed to 4 or 5 and see if it still happens. If so, is the treadmill rated for your body weight? Please be more specific.

Press the Stop and the Speed Up keys down while inserting the safety key. This should get you into calibration mode. Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. If you cannot get into calibration or if the EPROM test fails then the upper console will have to be replaced.
If you get pass to this point then proceed with the calibrating the incline and speed.
Press either incline key and the incline will calibrate automatically.
Be careful the treadmill will run at full speed in the next mode DO NOT STAND ON THE WALKING BELT.Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Press the Stop and the Speed Up keys down while inserting the safety key. This should get you into calibration mode. Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. If you cannot get into calibration or if the EPROM test fails then the upper console will have to be replaced.
If you get pass to this point then proceed with the calibrating the incline and speed.
Press either incline key and the incline will calibrate automatically.
Be careful the treadmill will run at full speed in the next mode DO NOT STAND ON THE WALKING BELT.Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Unplug unit remove motor shroud then start calibration procedure.
Press the Stop and the Speed Up keys down while inserting the safety key. If you cannot get into calibration then upper console needs to be replaced.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. At this point if it show 2F FF or Fail then upper console needs to be replaced.
Press either incline key and the incline will calibrate automatically
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON WALKING BELT.
Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled.
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing.
Press the Stop key once and remove the safety key.

Unplug unit remove motor shroud then start calibration procedure.
Press the Stop and the Speed Up keys down while inserting the safety key. If you cannot get into calibration then upper console needs to be replaced.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. At this point if it show 2F FF or Fail then upper console needs to be replaced.
Press either incline key and the incline will calibrate automatically
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON WALKING BELT.
Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled.
The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing.
Press the Stop key once and remove the safety key.

These units only have a calibration mode. The High end units generally will have that. If this is a Epic Then this calibration procedure should work.
Press the Stop and the Speed Up keys down while inserting the safety key. Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test. Press either incline key and the incline will calibrate automatically. BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON THE WALKING BELT. Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window). If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled. The max speed potentiometer is a small square with a movable center and is located near the three small speed wire connections labeled H, W, & L and is on the same edge of the board and the red and black motor wires are located. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments. It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure. THIS IS A VERY IMPORTANT STEP BECAUSE YOU WILL GET AN ERROR MESSAGE IF THIS STEP IS SKIPPED. After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

Press the Stop and the Speed Up keys down while inserting the safety key.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test.
Press either incline key and the incline will calibrate automatically.
BE CAREFUL, THE TREADMILL WILL RUN AT FULL SPEED IN THIS NEXT MODE! DO NOT STAND ON THE WALKING BELT. Press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled. The max speed potentiometer is a small square with a mobile center and is located in one corner of the board and is labeled MAX SPD. It is near the harness that carries the speed signal labeled H,W,L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments. It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure. THIS IS A VERY IMPORTANT STEP BECAUSE YOU WILL GET AN ERROR MESSAGE IF THIS STEP IS SKIPPED. After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing. Press the Stop key once and remove the safety key.

E21 means the treadmill is not picking up a speed signal once the start key is pressed. First, determine if the motor is turning before you get the E21 or not. If the motor turns, the speed sensor is the most likely problem. If it does not, you can manually turn the belt with your feet before you get the E21 and you should get a speed reading but eventually you will get an error code because it is almost impossible to keep the speed constant enough for the computer. Try running it through calibration with the motor shroud off.
Press the Stop and the Speed Up keys down while inserting the safety key.
Press the Stop key one time. The Time window should read EP:2P. Some newer models may also say FP, Pass, or the like. This means that it passes the EPROM test.
Press either incline key and the incline will calibrate automatically.
On this next step do not step on the treadmill when you press the Speed Up key and hold it down until 85 is shown in the Distance window (some models may show the 85 number in another window).
If the top speed of the treadmill does not match the displayed speed on the computer console, adjust the max speed potentiometer (pot) on the motor control board. This is the board that the red and black motor wires connect directly into in the motor area. The Max Speed pot is clearly labeled.
The max speed potentiometer is a small black square with a white center and is located near the three small speed wire connections labeled H, W, & L. Using a small insulated screwdriver adjust this potentiometer clockwise to speed up and counter-clockwise to slow down the max speed of the treadmill. A tiny turn of this potentiometer will make a big difference in speed so be careful when making adjustments.
It is not important to get the speed set exactly at the max speed of the treadmill but do make sure to get the speed within .2 MPH of the max speed of the treadmill. For instance if the max speed of the treadmill is 10 MPH, make sure to have the speed set between 9.8 MPH and 10.2 MPH. Some models have a 12 MPH top speed, so please check your manual if you aren’t sure.
After completing calibration of the speed, press the Speed Down key. The treadmill will automatically return to 0 MPH. Please allow the unit to stop completely before continuing, press the Stop key once and remove the safety key.
If the motor does not run, test the motor but first unplug unit then unplug the two leads ( Red and Black ) that is connected to the Motor Control Board.Cordless drill battery- get a fully-charged, 18 volt (preferred) cordless drill battery and on most treadmills connect the red and black motor leads to the battery (some have different colors but if you have two wires and get it hooked up backwards, the motor will just run backwards and won’t be harmed). If good, the motor should operate at about 1 MPH. Then Motor Control Motor will need to replaced

If you have a unit where the speed can be adjusted with 1 mph increments 0 to 10 and with speed up or down .1 mph increments . it sound like the key pad itself is causing this problem. Depending on manufacturer you can purchase the keypad/overlay and some will want you to purchase the entire display.

You should have a speed arrow up & down, this increases and decreases the speed in .1 mph. Try this control and see how the unit re-acts if the speed does not adjust in .1 mph increments or it stay the same either way it sounds like an upper board issue. Go to iconfitness.com go to customer service the replacement parts put you M# and will give part# and pricing.
RULE OF THUMB: IF REPAIR IS MORE THAN THE THE PURCHASE PRICE OF UNIT DOE NOT REPAIR