Shimano Ultegra R8000 L-Shifter "Fail"

 I've run into a problem with my L-Shifter, the one that shifts between the big-ring and small ring on a 2x11 gearing. Given how hard it is to buy these, vs the right shifter, I think this is a common problem. 

It's very hard to examine the internal workings of the ratchet mechanism, but it behaves as though the "H" ratchet tooth's back-side is shaped differently somehow, so that when you shift past it and then relieve when letting the shift paddle fall back, SOMETHING inside the shifter malfunctions, or at least, acts in an aberrant manner. My guess would be the half-round bit that straddles the ratchet and implements the 1/2 step relief back to H- gets stuck or hung-up because the shifter then refuses to shift from H- back to H.

The problem is hard to define, but easy to fix once you are aware of it. Here is the explanation I sent to my "Gear Head" at Competitive Cyclist where I bought my Pinarello. They sent me a new set of shifters 2nd Day Air at their own expense because Shimano is so backed up it would have taken 2 months. For some reason, they sent me a Di2 shifter set, which is hard to reconcile with the explanation below, but I give them an A for effort, and the Gear Head thing really is nice in situations like this, as I already have a return label to send the Di2 back.

The Shimano Ultegra R8000 L-hand shifter has developed an odd shifting problem.

As you probably know, there are 4 shifting positions for this shifter, let's call them

H
H-
L
L-

I should be able to shift from any of those into any of those, and could until a few weeks ago. Now, however, I can shift from L to H, and relieve back to H- alright, but once I am on H- I  cannot shift back into H. I have to drop down to L, and often L- to shift into H. More and more now I cannot even shift from L to H. I have to shift all the say down to L- in order to shift into H. Otherwise the shift lever simply refuses to budge, making this a Go/No-Go failure.

I removed the cover from the shifting mechanism on the shifter (oddly, the retaining screw was ~ half unscrewed, so maybe the assemblers had encountered some problems with this too) and inspected it. I can't see any debris, broken wire strands, grit or dirt in the mechanism. In fact, it looks very clean. I also inspected the front derailleur, and it too looks like it's in perfect shape.

There is no reason I know of why this mechanism should be failing, and I don't think there are any "user serviceable parts" on the shifter, aside from cable replacement, so I have to think something inside the shifter was defective upon mfg and the shifter needs to be replaced.

After I sent this, I did another Google Safari, and after about an hour came up with just one poorly described definition of the problem and solution, so I think this needs a little better airing.

It appears that Shimano designed the shifter so the H-Stop set-screw needs to stop the travel of the cage, NOT the shifter's cable travel. To me this is an appalling design flaw as even normal cable stretch - which is what happened in my case - will eventually overcome the barrel adjuster's ability take up enough slack/tension (it really IS tension, not travel in this case, as the cable must NEVER be even a tiny bit slack when over the big ring)

My RX is to back the cable tension adjustment screw on the derailleur AND the barrel adjuster off as far as you can to give you as much slack as possible to get as much cable slack as necessary, and then unscrew the cable clamp screw and pull up all the slack you can before tightening it back down. The little plastic thingy is a bit of a PITA but does make a nice aero-clean bit when all assembled properly again.

Adjust the H & L stops to arrest travel, and use the cable tension screw (put some LockTite on it too, another appalling oversight on Shimano's part) to tweak the derailleur position.