alpha8,

You can try to see if resetting the unit will resolve the problem:

1) Disconnect all cables except for a power supply cable (if using a power supply)

2) Press and hold the 'A' and 'D' buttons down AS you power on the FM4 by plugging in a 1/4" cable into the Left/Mono input.

If you continue to have problems after a reset, you will need to take the pedal to an authorized service center:

http://www.line6.com/support/serviceCenters/

Regards,

Line6Hugo