Hi,
It is rare to have a malfunction and have not heard of it being out of action since.
Last year when travelling to LAX from T5C . I got on the shuttle at T5A got to T5B and pax were let on and the train Returned to T5A!

where we had to reclear security again ( understandable but :mad) and then walk along the tunnel to T5B and then T5C ( that is a long walk!

)
Regards
TBS