Where is it? Located on the front face of the inlet manifold

What is it? The MAP sensor
measures the pressure or
vacuum level in the inlet
manifold. It works by converting internal diaphragm deflection into a 0-5Volt output signal(see signal here).

What's it for? By measuring the manifold absolute pressure (Atmospheric Pressure - Manifold Pressure) the ECU can calculate the Air Fuel ratio. The ECU makes the assumption that the engine will draw the same volume of air per revolution, it therefore requires to know the RPM for this calculation ( Crank sensor). Because the air density changes with temperature the ECU also measures the air temperature entering the engine at no 4 cylinder ( Air Temperature Sensor). Because the inlet manifold is "dry" ie. injection occurs at the back of the inlet valves, no fuel trap is required, as is the case on some K series models. On some roller barrel throttle body models there is a problem at higher altitudes where the air is thinner, this has recently fixed by using a compensation unit.