Getting it from the manual ..Is the manual incorrect ..I dont own a FL8 of any model so no hands on experience
https://drive.google.com/drive/folders/0B6O6hqT9O0J6OVY5bnRRMjRWWTQThe yellow button in the middle of the panel controls the Interference
Rejection feature. The red LED to the upper left of the I.R. button tells
you if the rejection circuitry is on. The I.R. feature has eight steps of rejec-
tion to let you eliminate unwanted interference from another nearby
depth finder. When you turn on the FL-8SLT on, the I.R. automatically
comes on to the first step. If necessary, press the I.R. button repeatedly
until the interference is eliminated. You can press the button seven times
before you get back to the first step again. Turning the unit off and then
on again will also reset the I.R. back to the first step. To turn the I.R. fea-
ture off press the Gain control knob. The red LED will also go off.
TIP - If running two SLTs, it is best to turn the I.R. off on one unit
and then eliminate the interference on the other unit.