1.6 gal Koehler, tall style, has stopped filling completely. This toilet is about 3 years old and started not filling completely this week. No changes have been made to plumbing. There is no indication of any leaking with in the toilet.

You mean low water level in tank, not bowl, right? If so, sounds like you merely need to adjust the fill valve float.

Since the title says low water level in the BOWL then my first thought is that the fill tube in the toilet tank has popped out of the overflow pipe and needs to be set back in place... ;)

Here, thomashcoman, there is a refill tube that runs from your fill valve (in toilet tank) to the overflow pipe in the center of the toilet tank... feeds water into the bowl as the tank fills (see image)... ;)

Lift the lid to the tank and check to see if the refill tube is dislodged. If so, replace it to the angle adapter or hole/slot and that should resolve the issue.

If MGD77 is correct and you really have low water inside the toilet tank and the toilet isn't flushing properly then I think that you may have an issue inside the tank, such as a loose refill tube getting hung up on the fill valve float cup, or the fill valve washer is plugged up... maybe?

Anyway, post back with more info... would be helpful to us... and we'll see what we can determine for you, OK?

Mark