Can a Holter monitor diagnose life-threatening heart problems?

Absolutely!  Holter monitors continuously monitor the electrical activity of the heart for a variable period of time, traditionally 24 hours.  "Sudden cardiac death" and other serious heart problems are often caused by electrical abnormalities of the heart which would be picked up by the monitor.  Holter monitors can also be very helpful in diagnosing other serious electrical problems which, while not immediately life-threatening, have potentially serious consequences over the intermediate and long-term.

The shortcoming of Holter monitors is that often times electrical abnormalities don't occur in the relatively short period of time when it is being worn.