Is it possibly this (from the manual) switch on the lower far left corner of your POD?
Mode Footswitch/LEDs - Use this switch to toggle through the 3
available footswitch modes.
Preset Mode (Green LED):
A B C D
Press a footswitch to load its preset (A, B, C or D).
Looper Mode (Red LED):
Equips your POD HD400 with 24 seconds of looping capability.
The PRE/POST footswitch allows you to set your loop recording
and playback either before amp and effects modeling or after (LED
off=PRE, LED on=POST). Recording in PRE mode (LED off) let’s you
record your guitar, then audition various effects with your loop playing
back, while recording in POST mode (LED on) captures your amp and
effects in the loop and then layer different sounds by changing presets.
Note: changing the state of the PRE/POST footswitch while the loop
is playing back can dramatically change the volume, so be careful!)
Stomp REC/Overdub to start recording a loop then stomp Play /
Stop to end loop and immediately start playback. You can overdub
additional parts by again, stomping the Rec/Overdub switch. Stomp
Play /Stop to stop or start playback. You can exit Looper Mode even
when a loop is still playing to switch presets or turn amps and effects
on/off and return back to add additional loops. How cool is that?
Pedal On/Off Mode (Amber LED):
AMP FX1 FX2 FX3
Gain total control of your preset tone in Pedal On/Off mode. There are
4 components to your signal path - Amp, fx1, fx2 and fx3. Use the
appropriately labeled switch to turn any of these on (LED lit) or off.
Just like a real pedalboard full of stompboxes!