2003 anti theft

i have a 2003 malibu  and the anti theft system light is on  can  i use a code reader to diagnose it  and what could it mean     1 door lock isn't working     could fixing it take care of this  and thank you for any help

If this light is on your instrument cluster check this out it's likely related : http://www.chevroletforum.com/m_42526/tm.htm
 
A code reader wouldn't likely help.