Welcome to the site, HouBill!
We did some
and put your post into a new thread, since you have an issue with a different machine than the thread you originally posted in (which had already been resolved).
First,
NO, do NOT use your clear chip! That's a last resort, and I can't come up with a scenario in my head where a clear chip would resolve a coin-in optics issue.
Second, you have two (or more) machines -- great -- that'll be helpful for troubleshooting.
Okay, let's get started. Can you tell us what (if anything) you did or what happened leading up to getting a 21 error on your machine?
Next, when you close the main door, does the display go blank for a few seconds before the 21 returns, or does the 21 just stay on the display?
Finally, have you tried the optics from the problem machine on your working machine to confirm that the optics are still good? You don't need to change the comparator -- just the optics located directly below the comparator.
Let us know the answers to the questions I've asked you, and we'll be able to continue and eventually get you up and running! Again, welcome to the site.