Is the message "COIN-IN TIMEOUT"? Please be exact when giving error messages, as there are similar messages that mean different things.
Are coins being accepted into the machine (and into the hopper)? If so, your coin comparator is working fine.
The light on the comparator only comes on when it is enabled by the machine, which typically only would be when the door is closed. As such, you'll typically never see the light on the comparator come on.
The problem is likely in your coin-in optics, which are located directly below the coin comparator. The optics are encased in a small rectangular black plastic enclosure, and contain three pairs of emitting and receiving optics that are designed to detect that a coin has passed by and has gone through in the correct direction. Here is what the insides of the optics assembly look like:
You'll want to check the self-test input screen to see what the three coin-in optics (COIN DETECTOR A, B, and C) are reading, and if they respond correctly when a coin is inserted. Open the door and press the white self-test button once, it should bring you to this screen:
In their normal, idle, working state they'll all show "1" indicating that the optics are working and not blocked. As a coin passes through, they should quickly switch to "0" as the optic is blocked, and then back to "1" once the coin passes through.
Use a popsicle stick, or a thin piece of cardboard to block the optics one at a time (you may need to remove the coin comparator to access the top of the optics stack). Ensure that the optics respond as they should on the self-test input screen, switching to "0" as they're blocked, and back to "1" when they're not. Let us know the results.