A MAP sensor or manifold absolute pressure sensor senses intake manifold air pressure. The PCM uses this information along with information from other sensors to adjust the air-fuel ratio. A faulty MAP sensor will cause an engine to surge while cruising and stall at idle.
They look like a small black box located near on or near the intake manifold and attached with a short vacuum hose. This hose and intake manifold's port should be inspected for blockage that can corrupt the signal. The port can be checked with a vacuum gauge; intake manifold vacuum is 18 "hg to 21 "hg.
The sensor is used as a barometric pressure sensor, informing the PCM of pressure conditions at key ON. It tells the PCM of air pressure by altitude/humidity. Air has less pressure and is thinner on a mountain than on a beach.
Testing these sensors is best done with a scan tool. Manufacturers provide resistance and voltage testing information for this sensor. Check with these specifications before proceeding. A special testing device may be required with some models.