Start with the coin optics below comparitor.
NOW, assuming you get error 21 on startup. If you get error 21 after you dropped a coin, let me know!
Error 50 means door is/was open. Error 13 means power interruption(you turned it on). No TILT for these two, should always get them when powering up (with door open-power supply switch.)
Error 21 is Coin-in jam. This is your TILT.
With power on check the two optics below the comparitor. They both should show a little red LED lit. Sounds like one or both do not.
Make sure both are plugged in. Ckeck for paper/dust or any obstruction in the optic. Check optic alignment where it is mounted to make sure optical path not blocked.
If you find that one is bad, there should be a third optic down lower for coin drop to bucket in the slot stand. You can use this optic as a spare and replace the bad optic.
Now, if error occurred only when coin was dropped, it could still be bad optic or someone messed with the optic timing in the test mode.