This paper addresses MEMS testing through a case study: a micromachined magnetic field sensor with on-chip electronics. The sensor element is based on a cantilever beam that is deflected by means of the Lorentz force. Embedded piezoresistors are used to detect strain in the cantilever beam and thus to detect the magnetic field. A test approach is presented for the whole system focussing on fault classification, on design for testability and on production test costs. Fault classification introduces several catastrophic and parametric faults on both mechanical and electrical elements. Simple and low-cost design for testability such as test point insertion is then discussed for test cost reduction and for fault coverage enhancement.