Shimano GRX / Ultegra STI Lever - Faulty

My rear GRX 810 shift lever seems to be hitting an obstruction when I try to shift up out of the hardest gear (smallest cog). I can shift up once and then usually the lever just seem to hit a hard stop like there is some obstruction in the mechanism. It's not like there is some frayed cable, dirt or grease there. Also, randomly it will work perfectly for a bit along the whole range. Then it will revert. I've taken the cable out and removed the plastic cover at the bottom of the lever, but looking inside I can't see what the obstruction is.

Any suggestions? I can't see anything online about maintaining this part of the shifter so guessing it's not even designed to be disassembled and repaired. Could be a warranty claim as it's about 16 months old.

Hi my friends left grx shifter had a similar issue we found that the spacer at the back of the lever had fallen off ,
the spacer is located next to the paddle
It’s a long shot but just a maybe

Cheers, could be that. Day before had the bike upside down doing some cleaning so may have fallen out then

Hey I’d be interested to know if this the issue with your shifter cos I’ve never had this part fall off any other shifter that I own or have owned, wondering if this is a fault with GRX shifters

M neither. Doesn't inspire confidence.

I've heard from a mechanic that the issue is not unheard of with these shifters (he hasn't inspected my shifter though).

Will look to warranty it and will update once I get diagnosis.

So, 5 months on, warranty claim was successful. Retailer received credit from Shimano and has passed it on to me. It's at trade amount, so doesn't compensate for what I had to pay to get a new shifter, but it's something. They didn't say what the fault was.

Had exactly the same thing happen with a Ultegra STI lever. This one over 2 years so out of warranty. Anyone had similar experiences/ suggestions for how to fix?