There are two main types of air monitoring devices: mass air flow sensors (MAF) and manifold absolute pressure (MAP) sensors. MAF sensors measure the mass of air entering the engine and are either vane-style or hot-wire sensors. MAP sensors measure vacuum and pressure in the intake manifold. Together these sensors provide engine control units with critical data on air intake used to optimize fuel injection, ignition timing, and other engine functions. Common issues with air monitoring sensors include contamination, leaks, electrical faults, and degradation over time.