The Atmospheric Pressure Sensor is used by the ECU to determine the air pressure of the ambient air in or around the vehicle. The ECU uses this input to adjust air/fuel ratio and ignition timing for changing altitude conditions (and thus density of the air entering the engine). On most Honda's, this sensor is built into the ECU and not changeable. If you are receiving ECU trouble codes indication a faulty PA (pressure atmospheric sensor), most likely the problem lies with the MAP sensor and intake air temp system rather than the atmospheric pressure sensor itself. The ECU uses all of these in conjunction to determine the air density and volume entering the engine.