Key works perfectly outside, won't turn inside

[AVR2]

This lock has been getting progressively more unreliable, to the point where now we can't unlock it from inside, the key will only turn a few degrees. From outside, the key works as smooth as glass with no resistance whatsoever.

Is it fixable in a way that allows us to keep using the existing keys?

 

[blup]

It's difficult if not impossible to know.

You could try taking it apart, and using graphite spray to lubricate the components. But it might be down to wear.

Blup

 

[mattylad]

Does it do the same for all keys that work in this lock?

Try a squirt of oil in there first.
But I'd be tempted to strip it down and see if the lock is actually OK, else replace it with a new one as it will be far less expensive than however you will open the door when the lock finally fails.