The Mass Air Flow (MAF) sensor directly measures the mass of air being drawn into the engine. The sensor output is used to calculate injector pulse width. The MAF sensor is what is referred to as a "hot-wire sensor". The sensor uses a thin platinum wire filament, wound on a ceramic bobbin and coated with glass, that is heated to 417°F (200°C) above the ambient air temperature and subjected to the intake airflow stream. A "cold-wire" is used inside the MAF sensor to determine the ambient air temperature.
Battery voltage, a reference signal, and a ground signal from the PCM are supplied to the MAF sensor. The sensor returns a signal proportionate to the current flow required to keep the "hot-wire" at the required temperature. The increased airflow across the "hot-wire" acts as a cooling fan, lowering the resistance and requiring more current to maintain the temperature of the wire. The increased current is measured by the voltage in the circuit, as current increases, voltage increases. As the airflow increases the signal return voltage of a normally operating MAF sensor will increase.