Ambient Barometric Pressure Sensor The ECM incorporates an integral ambient barometric pressure sensor. This internal sensor is supplied with a V feed and returns a linear voltage of between 2.4 and 4.5 Volts. This represents the barometric pressure. The system monitors barometric pressure for the following reasons: l The barometric pressure along with the calculated air mass provides additional correction for refining injection ON time l The value provides abase value for the ECM to calculate the air mass being injected into the exhaust system by the secondary air injection system. This correction factor changes the secondary air injection ON time which in turn optimises the necessary airflow into the exhaust system l The signal is used to recognise downhill driving to postpone the start of evaporative emission leakage diagnosis.