TM 55-1510-222-10 will illuminate and not extinguish until the mode selector is set to OFF.  The battery unit should be replaced when this failure occurs. (c)   Interpretation   of   failure   indications.      It   is important    to    be    able    to    correctly    interpret    failure indications   in   order   to   take   effective   action.      Failure indications  are  listed  below  under  two  main  categories: WARN  light  illuminated,  and  WARN  light  extinguished. Under    each    of    these    categories    are    listed    other indications    which    will    give    the    operator    sufficient information to take action. 1. WARN annunciator illuminated. Take the following action: a. If   action   codes   01,02,03,04   or   05 are displayed - See table 3-2. b. No    action    or    malfunction    codes displayed - Indicates computer failure. c. Improper   displays   -   Indicates   NU computer failure. 2. WARN annunciator extinguished. If CDU displays are blank, incorrect or frozen - CDU failure is indicated. NOTE If  it  is  not  possible  to  load  displays from   the   keyboard.      A   temporary failure     of     a     numerical     key     may prevent   data   loading.   If   a   number cannot   be   loaded   into   latitude   or longitude displays, after pressing/wiggling    the    key    several times, the cause may be the momentary  hang-up  of  another  key. To  identify  the  faulty  key,  rotate  the data   selector   to   DSRTK/STS.      The right     digit     in     right     display     will indicate   suspect   key.      Press   and release   suspect   key   several   times. To test whether the keyboard problem   is   corrected,   try   pressing any other numerical key.  Its number should now appear as the right digit. If  this  test  is  successful,  press  the CLEAR  key  and  return  data  selector to    original    data    loading    position. Otherwise, a CDU failure is indicated. Table 3-3.  Malfunction Code Check Step Indicator or Indication Control Operation Display Indication 1 WARN light Lights 2 Data Selector Rotate to DSRTK/STS RH data display Action   code   second   and   third digits 3 TEST switch Press and release RH data display Lowest number malfunction code which   has   occurred   since   this procedure was performed replaces action code. 4 Repeat Step 3 repeatedly, recording all malfunction codes until second and third digits again indicate an action code or go blank.  Refer to Table 3-4 for action codes and recommended action and to Table 3-5 for malfunction code definition. 5 If  WARN  light  extinguishes  and  two  digits  go  blank,  failure  was  intermittent  and  has  been  cleared.    If digits do not go blank, perform action according to displayed recommended action code. 3-65