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Located on the right side of the upper plenum is the GM-style MAP sensor. MAP stands for "manifold absolute pressure." This particular MAP contains a vacuum nipple, which goes into the plenum, sealed by an O-ring. Internally, the MAP sensor contains a vacuum sensitive potentiometer, thus sending a signal based off of a five-volt reference to the MEFI controller. At idle, vacuum is high; under load, vacuum decreases. Based off of the readings of the MAP sensor, throttle position sensor, and oxygen sensor, the computer can calculate and correct necessary variable outputs for the timing and fuel injector pulse.