I got this off the forum , I think Merlin posted it.

If the Onboard expression pedal of the POD HD500 does not sweep through parameter values as it normally would, you may need to perform a pedal calibration to reset the functionality of the expression pedal:

Hold the down the RIGHT ARROW button on the 4 Way Nav Pad while powering the unit on, until you see the “Test Mode” screen (Pedal Cal, etc.), and then proceed with the steps below:

1.   If not already selected, use  on the 4 Way Nav Pad to select “Pedal Cal,” as shown above.  

2. Press the VIEW button.

3. Set the pedal to the minimum “heel” position, then press the A footswitch to set a value for the Duty Cycle. You’ll see the value set to “100.”

4. Set the pedal to the maximum “toe” position, then press the B footswitch to set a value for the Duty Cycle. Again, you’ll see the value set to “100,” as in step 3.

5. Press the C footswitch to prepare for automatic selection of the Scaled Pedal Value and move the pedal from min. to max. You should see the Scaled Pedal Values read “0” at the heel position and “255” and the toe position. If you get values other than 0 and 255, power off and repeat these steps, starting at step 1.

6. Press the D footswitch to save the settings. You’ll be returned to the Test Mode screen and the Pedal Cal item will show a “P” to indicated that the pedal calibration is complete! Power your device off and back on again to start using it.