You mentioned that you have already replaced the Laser Unit, but the error persists. Here are a few more steps you can try:
Check the cable: Ensure that the cable connected to the Laser Scanner Unit is properly attached and not damaged.
Replace the DC Controller PCB (PCB4): If the error still persists after checking the cable and replacing the Laser Scanner Unit, you might need to replace the DC Controller PCB (PCB4)
Here are some YouTubes that may help - once you get past ads.
Exciting New Discovery (youtube.com)
How to fix error code E110 in Canon ImageRunner Cantina IR
2520 2525 2530 #canon #subscribe (youtube.com)
If your Canon iPF605 printer won't enter service mode and gets stuck at the "S" symbol without initializing, it may indicate a firmware or hardware issue. Consider updating the firmware or contacting Canon support for further assistance.