It is the sensor that is in the air inlet tube, behind the air filter, that leads to the throttle body.

It looks square from the outside and is held inplace by two screws. When you take it out, there is a very fine, thin piece of wire. See if it is dirty or broken. If dirty, parts stores sell MAF sensor cleaning stuff. See if that works. $3 can of cleaner is better than $400 for a new sensor.

S.