Cardiac resynchronization therapy is a non-pharmacological treatment for patients with dilated cardiomyophaty and congestive heart failure. The success of this therapy depends of permanent biventricular stimulation. We report an 84 year-old man, with intermittent loss of biventricular pacemaker stimulation despite having adequate sensing and stimulation thresholds in the right atrium and both ventricles. The problem was solved after correcting some programming parameters