While cleaning my bike this afternoon, I removed the chain and cassette to clean them. This is a 2018 Trek that I have done this many times to with no issues until today. After putting it back together, I could no longer shift to the small sprocket. It just stops on the sprocket before it like that is the end of travel.

I did not adjust any limit screws or barrel adjusters, the lock ring was torqued to 40nm. Prior to this, there was some noise like it was having some trouble getting onto that sprocket, but it would get there. It has probably been a couple of years since the shift cable was replaced so I will order a new one and it is probably time for a chain too. But I just thought it odd that this occurred after maintenance and wondered if anyone had any other thoughts.

The fact that it was "almost" doing the same thing before you "serviced" it suggests that the issue was already present, so I would say limit screw and/or cable tension, and possibly also cable drag in the housing. Also, if you removed the cassette, check to make sure the smallest cog is sitting correctly in there, you'd be surprised how often DIY repairs come in with one sitting cockeyed, backwards, or a missing spacer.

wheelreason Thanks. I thought about cable tension last night before I went to bed. I think this may be the main culprit, and if so, I should be able to correct it when I replace the cable.

And I don't mean to sound like a retro-grouch, because I'm not. But my friction shifting Suntour derailleurs on my old bikes are no problem.

Why not just loosen the cable tension a little and check the limit screw now before installing a new cable to find out if this solves the problem. Only takes a few minutes.

Small rear sprocket as in physical size and tooth count or small rear sprocket as in smallest gear ratio?

If the former, then I'd definitely pull the cable out of the STI... or attempt to pull the cable out of the STI and see if it's frayed.

Sometimes when you do mechanical maintenance on one thing, Murphy will makes something else simultaneously go phooey that isn't directly caused by what you worked on. Or perhaps Murphy has blinded you to something you are actually doing wrong when you put it back together.

Examine the cable closely where it anchors in the shifter - this is where it often starts to fail, and you really don't want it to break off in there. If it looks healthy check that the cable casing is cut square (no strands escaping) and properly seated wherever it stops, then in bottom gear give the longest exposed part of the cable a firm sideways tug to make sure it's bedded in. It might just be time for a new cable.

I'm going to be in Tulsa this afternoon so I'll pick up a new cable otherwise I would just loosen the clamp and go on. While I've never had this cable fray, I understand it is a common issue and I like to do this preemptively as a result. For my ride this morning, leading a local 5K it was fine and I wasn't going to mess with it.

there's a chance that, while cleaning, a cable end got pulled out of the housing seat on the der. or shifter... this would move the cable about 1 shift or so more taught......

I checked that because one time the plastic barrel adjuster broke on this bike and caused issues. I meant to get to a bike shop yesterday and get a cable, but unseasonably hot weather made sitting at my grandson's football game miserable so we just went on home. I'm afraid that the prior shifting issues and this one may be a sign that the cable is frayed in the STI. I rode an old bike for a spin today.

I understand that you may be a perfectionist, but do you ever ride on the smallest sprocket? If so, how often, and what % of your riding is done on the smallest sprocket?

I have two geared vintage 14-speed bikes that also do not shift to the smallest sprocket, but I never ride on it. I do all my riding on the 3rd and 4th sprockets from the smallest one; those two sprockets are my favorites—one for flat roads and the other for inclines. Since I never use the smallest sprocket, I don’t bother fixing it at all. One less headache.

There is nothing perfectionist about shifting into all of your gears. That is just a bike working normally. People need to stop acting as if making a bike function as it should something more than that, it is not perfectionism it is just how the bike should be.

I wouldn't consider myself to be a perfectionist, but I've been an industrial controls technician for over 40 year and correcting deficiencies is just what one must do to ensure reliability. And as veganbikes mentioned, it is just making sure that everything functions properly. Because if that cable is frayed, I will certainly be using the small sprocket even if I don't want to or making some make do repair to get home. And knock on wood, I haven't had to pick pieces of cable out of an STI, but I have read it can be a pain.