Theory of Operation

The Mass Air Flow Sensor is a frequency based device. A constant voltage is applied to a heated wire. This wire is positioned in the air stream and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (ptc) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency which is sent to the PCM and interpreted as air flow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film. This diagnostic has two parts, a key on monitor and a monitor that runs in operating conditions when the EGR valve is closed. The key on monitor diagnostic fails if the Mass Air Flow Sensor reads a non-zero value at key on, engine off. The engine running monitor compares an estimated charge air flow value to the Mass Air Flow sensor reading. This monitor only runs when the EGR valve is commanded closed. During this time the MAF sensor value should equal the charge air flow estimate. The PCM will light the MIL lamp if this diagnostic runs and fails in two consecutive drive cycles. The PCM will turn off the MIL lamp when the diagnostic runs and passes in four consecutive drive cycles.