The manifold absolute pressure sensor (MAP sensor) is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). The data is used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion (see stoichiometry) and influence the advance or retard of ignition timing. A fuel-injected engine may alternatively use a mass airflow sensor (MAF sensor) to detect the intake airflow. A typical naturally aspirated engine configuration employs one or the other, whereas forced induction engines typically use both; a MAF sensor on the Cold Air Intake leading to the turbo and a MAP sensor on the intake tract post-turbo before the throttle body on the intake manifold. MAP sensor data can be converted to air mass data by using a second variable coming from an IAT Sensor (intake air temperature sensor). This is called the speed-density method. Engine speed (RPM) is also used to determine where on a look up table to determine fuelling, hence speed-density (engine speed / air density). The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines. Example The following example assumes the same engine speed and air temperature in a naturally aspirated engine. Condition 1: An engine operating at wide open throttle (WOT) on top of a very high mountain has a manifold pressure of about 50 kPa (essentially equal to the barometer at that high altitude). Condition 2: The same engine at sea level will achieve that same 50 kPa (7.25 psi, 14.7 inHG) of manifold pressure at less than (before reaching) WOT due to the higher barometric pressure. The engine requires the same mass of fuel in both conditions because the mass of air entering the cylinders is the same. If the throttle is opened all the way in condition 2, the manifold absolute pressure will increase from 50 kPa to nearly 100 kPa (14.5 psi, 29.53 inHG), about equal to the local barometer, which in condition 2 is sea level. The higher absolute pressure in the intake manifold increases the air's density, and in turn more