So I went out today and after about 2-3 miles without any drama I couldn’t shift down to the lowest gears.

Only a few weeks old.

Any ideas or pointers what to try first

Cable snagged somewhere?
A section of outer wasn’t seated properly and has dropped in to the guide?
Inner cable wasn’t seated properly in the shifter and has moved?

Disconnect the cable from the derailleur and see if you can move it by hand to the lowest gears. If you can then it’s either a cable problem or a knackered shifter.

Cables 90% of the time

Undo the cable from the mech, pull tension on it by hand and shift through the gears to see if it’s smooth and free for each gear. If it is then it’s prob a mech setup issue. If it’s not change the inner / outer cable.

I found 12sp SLX needs very little tension on the cable when you are tightening the mech in the bolt to get smooth shifting.

Check the B limit is correct and check the H/ L bolts are set correctly.

If you aren’t sure how to do it, download the Shimano dealer manual from their site for your mech and it will talk you through it.

Clutch mech(?) with clutch set to stiff?
Does it shift well with clutch off?

Clutch mech(?) with clutch set to stiff?
Does it shift well with clutch off?
(Edit…although why it should suddenly happen after a few rides would be odd)

thanks for the replies i will investigate now.

it previously shifted well with clutch on and off as my first ride the clutch wasn’t in the on position (from when i picked the bike up) and i only realised as i got alot of chain slap, then it was in the on position from then on.

I did notice in the lowest gear the chain seemed a bit slack so that was meant to be my job for yesterday but the sun was shining so i thought id get my “hour exercise” and then this happend..

cheers

Outer cable not fully seated into ferrule, after a bit of use it tightens up and that leaves slack in the inner.

Just to clear up any misunderstandings, is the low gear that you’re refering to the smallest, or largest sprocket? Getting the terms wrong can lead you, and everyone else up a blind alley.

Make sure you’re not taking too much chain out if you’re shortening it. follow the manual, depressurise or remove the shock to find the longest chainstay distance if it’s a FS.

Reasons above probably cover it. Or if you just mean not shifting well it could be just cable bedding in and needing a tweak on cable tension, or bent hanger.

Cable not clamped in the mech tightly enough and has pulled through a bit .

Replaced the cable and reindexted and now all is good