Here, we report the case of a 59-year-old man with cardiac sarcoidosis (CS) who experienced the failure of a cardiac resynchronization therapy-defibrillator (CRT-D) because the sensing lead failed to operate correctly. The sensing lead, which was inserted into the right ventricular outflow tract (RVOT), failed because of a newly emerged intraventricular conduction abnormality (IVCA) resulting from the progression of the CS). For determining the cause of the failure, an electrophysiological study was conducted with catheters placed to the right ventricle apex (RVA) and RVOT. The IVCA was not seen during rapid pacing at 180 bpm but was observed during rapid pacing at over 190 bpm; however, this phenomenon may also develop as a result of the restitution property of conduction velocity due to the relative ventricular refractory period, even in the absence of CS. A voltage map recorded by the CARTO mapping system revealed a markedly low voltage area between the RVA and RVOT. Therefore, we assumed the heart rate for the ventricular tachyarrhythmia was underestimated by the CRT-D sensing lead and had thereby led to its operation failure. An appropriate intervention was accomplished by inserting an additional sensing lead to the RVA, and the same phenomenon has not occurred to date. To our knowledge, no similar report has previously been published, and thus makes this case an extremely rare and didactic case.Key words: cardiac sarcoidosis, CRT-D, under-count, ventricular tachyarrhythmia.