An apparatus for determining and/or monitoring at least one process variable of a medium in a container. The apparatus has at least one mechanically oscillatable unit unit which produces a measurement signal at least dependent on the process variable. It also has at least one electronic unit which evaluates the measurement signal, and at least one function test unit associated with the electronics unit. The measurement signal passes through the electronics unit on at least one processing path, and the function text unit operates on the processing path during a function test in such a manner that a test measurement signal results from the measurement signal received by the electronics unit. The test measurement signal corresponds essentially to the measurement signal, which occurs for a predetermined value of the process variable.