MSA ALTAIR 4X Operating Manual page 26

When gas reading reaches 100% LEL or 5.00% CH 4 , the instrument
enters a Lock Alarm state, the combustible sensor shuts down and
displays "xxx" in place of the actual reading. This state can only be
reset by turning the instrument OFF and ON in a fresh air environment.
A combustible gas reading of "100" or "5.00" indicates the
atmosphere is above 100% LEL or 5.00% vol CH 4 ,
respectively, and an explosion hazard exists. Move away
from contaminated area immediately.
In such cases, the instrument LockAlarm feature activates.
2-14
" WARNING