It’s a shimano on a mtb. I can change up ok but when i pull on the down lever there is no click a none of the normal resistance in the lever.

So is it a lever problem a cable problem or the derailleur itself?

Thanks

Sounds like a gunked up cable. Replace the cable/outer and it should be fine. Does the derailleur move ok without the cable clamped?

No the derailleur won’t move at all. It’s on the biggest ring right now. I’ve put some oil on the cable. It’s routed under the frame and the inner is exposed in places so gets muddy.

You say you can change up OK. How do you know if you can’t change down (so that you have the chance to change up)?

But if you’ve not cleaned it all out, and tried the mechanism without the cable connected, those are a couple of things to go at first of all.

I had this the other day. I washed the bike and carefully lubed the front mech at the pivots, undid the cable and gently moved the mech across its throw until it was moving smoothly again, then reconnected the cable and adjusted the tension. All good now.

Do you know how to set up an FD?

if you do (or are happy to just follow the Park Tools guide) then undo the cable and check the FD is okay and the shifter works okay. Pull on the cable to put it under tension and flick the shifter down to the smallest ring and then set follow the set up guide.

I’ve undone the cable and it was a bit sticky in the outer. The mech is moving. I’ll set it up again and see how it goes

I cleaned and lubed it set it up again and it soon stopped working. I’ve fitted a spare Deore shifter and cable I found in the shed and all is fine now. I’m glad it went wrong today and not half way through my WHW adventure!

Thanks.