Brother HL-2400C Service Manual page 284

L1 (Beam Detecting Error)
Error Code
Scanner Unit
Concerned Part
Check the connection of the scanner unit connector.
NO
Is the connector PDCN properly connected ?
Connect the connector properly.
Are the connector LCN1 and LCN of MCTL properly connected ?
YES
Is L1 error cleared ?
NO
YES
Replace the scanner unit.
YES
Is L1 error cleared ?
NO
Finished.
Replace the MCTL PCB.
MCTL
PD LDU
SGND
PDCN
Optical Unit
LCN +5V-1R
BDT
LCN1 SM
SM
LCN3
20