Brother HL-3450CN Service Manual page 307

CHAPTER 8 TROUBLESHOOTING
Error Code L1 (Beam Detecting Error)
Concerned Part Laser Unit
Confirm the connection of an laser 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 laser unit.
YES
Is L1 error cleared ?
NO
Replace the MCTL PWB.
Job is complete.
MCTL
PD LDU
SGND
PDCN
Laser Unit
LCN +5V-1R
BDT
LCN1 SM
31
SM
LCN3
20
8-44