I completely rebuilt my truck engine, the only thing I didn't do was take the block out. But I replaced all the gaskets and piston rings. Took it apart and reassembled down to a bolt. Then I drove it 45 mi and the CEL turned on. Got home and plugged the OBD2 to read P0101 -- mass air flow. One thing I did not do was replace the air filter because it looked pretty clean.

How can I go about pinpointing what the cause of this code is? I understand what it means is that the engine is not absorbing the expected volume of air but the causes can be different, such as leaky intake manifold gaskets, which I replaced. Could the sensor itself be broken and need replacement?