The theory and operation of any mass airflow sensor is pretty basic. What you have is a wire in the Airstream entering the engine. It has current running through it. This current heats up the wire. As the air flows over this wire, it changes the resistance in the wire. The change in resistance shows the ECM, how much air is entering into the engine. But by heating up a wire that is in the Airstream entering the engine, even after the filter, it can have contaminants that can stick to that hot wire and cause faulty readings about the air entering into the engine. This can cause numerous codes because either it's under or over reporting the air entering the engine, and it's not very accurate. And using the oxygen sensor, the ECM can see issues with the mass airflow sensor. So what can you do both for maintenance and for service and diagnostics to confirm a mass airflow sensors functioning properly?