Its operation is based on supplying 5 volts of DC power to the sensor from the PCM (Power System Control Module). Inside the MAP sensor is a resistor that moves in response to the intake manifold pressure. The resistor alters the voltage between 1V and 4.5V (depending on engine load), and the voltage signal returns to the PCM to indicate manifold pressure (vacuum). This signal is required by the PCM to determine fuel delivery and is occasionally used to determine whether the EGR valve is functioning properly.
The Manifold Pressure Sensor is useful for diagnostics because it measures throttle performance, turbo performance, and leaks in the inlet manifold. Due to its position, it should always read negative pressure unless the turbocharger is boosting pressure. This is depicted in the diagram below.
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