The objective of this paper is to present a cost-effective fault diagnosis methodology for flash memory. Flash memory is enjoying a rapid market growth. The research for flash memory testing is mainly to reduce the test cost and improve the production yield. In this paper, we propose a fault diagnosis flow for flash memory. We also propose a flexible built-in self-diagnosis (BISD) design with enhanced test mode control, which reduces the test time and diagnostic data shift-out cycles by using parallel programming and erasure and employing a parallel shift-out mechanism. The area overhead of our BISD circuit is only about 0.5% for a 256Mb commodity flash memory chip. Experimental results from industrial chips show that the proposed diagnosis methodology has high accuracy in distinguishing the fault type.