1. A manifold absolute pressure (MAP) sensor emulator for diagnosing the condition of a MAP sensor by emulating the operation of the MAP sensor when normally functioning, for use with a vacuum probe coupled to an intake manifold of an internal combustion engine and generating electrical signals in response to vacuum pressure at the intake manifold, said emulator comprising: a user interface for inputting data representative of the MAP sensor to be emulated; and an emulation circuit, responsive to the electrical signals and to the data for automatically generating a sensor substitution signal representative of the normal operating output of the emulated MAP sensor, wherein said emulation circuit includes means for varying a voltage level of the sensor substitution signal proportional to a change in the vacuum pressure sensed by the vacuum probe.