BARO sensor measures barometric pressure of atmosphere. Variations in atmospheric pressure are converted to electrical signals and sent to PCM. The PCM uses this signal to adjust fuel mixture due to changes in altitude. The BARO sensor affects air/fuel ratio, EGR flow and spark advance.
The BARO sensor looks the same as Manifold Absolute Pressure (MAP) sensor, except the tubing nipple on BARO sensor is vented to the atmosphere and MAP sensor is connected to the intake manifold.