I have found the letters A,B,C and D, corresponding to the wires on the MAF but what is what ? There are only 4 wires on this type of MAF.

Here are the common problems:
leak between the MAF and the throttle body;
leak in the PCV system, especially the hose elbow;
leaking intake manifold gaskets;
any other vacuum leak;
"lazy" oxygen sensors (the front ones).
Is this what these’s code’s mean?
Thanks