I show how to easily fix many indexing type shifters that won’t shift. In this case it’s a road bike STI brifter, but it also works for mountain bike trigger shifters as well. Where the shifter just isn’t catching. The pawls get stuck with old grease. I just clean out the old grease with WD-40 and then lube the shifter with Tri-Flo. It only took a couple minutes to get the shifter working like new. I have heard of bike shops telling people the shifters were broken and needed to be replaced, but the people tried this method and got them working again. Though sometimes the shifters may be genuinely broken. But often than not, they are just gummed up.
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