The expression pedal on the XP series effect pedals can sometimes lose their calibration settings. This is usually caused by stepping on the expression pedal harder than normal. This will move the expression pedal farther than it was originally calibrated at. When this happens, the pedal needs to be recalibrated. The display will show "Pb" which is asking you to place the pedal back (toe up). After placing the pedal back, press the bypass button once. The display will change and show "Pf". This is asking you to place the pedal forward (toe down). Again, after rocking the pedal forward, press the bypass button once. Finally the display will show "SA" indicating that it has saved the calibration. The XP will return to a normal operating mode after this procedure has been performed.