Special thanks to NLG member Jim for sharing this information, and members Stayouttadabunker and newkid3 for the pictures.
On an IGT S+ machine, the coin-in optics may fail with optic 1 registering as always closed, even when its path is blocked by a coin. This can result in an error 21 (coin-in error), or inserted coins not properly giving credit.
Removing a specific transistor on the coin-in optics board may fix this issue for you. As always, do any work on your machine with the power turned off. There are two revisions of coin-in optics that were used on the IGT S+ machine. The transistor in question will be labeled as Q2 or Q4, depending on the board you have. Note that one of the coin-in optic emitters is labeled as Q4 on the "Q2" version of the board -- do NOT remove that emitter! Look at the attached reference images to determine which style of board you have and to see which part needs to be removed (note you must be a registered member (free) and logged in to see the images attached to this post).
You may use a small needle-nosed pliers or other similar tool to carefully remove the transistor, while taking care to avoid damaging any other components. Once the transistor is cleanly removed, place the coin-in optics assembly back into your machine and test it out.