AIR and MAF use same 02 sens feedback for diagnosis, so when a code 4 on DM comes up, they recommend going to HFM/SFI module for further specific storage. Pin #8 [ homemade tool]
That is where they separate the inputs

To explain it in simple terms, the codes that get to the DM to trip the CE light actually come FROM the other modules through a Can wiring system.. so, usually, once the CE [DM module] gets its info, it gets it LAST from the other modules fault detection . It reports these faults back to the DM when it is determined that these faults will effect Emmission Mandates..and that is what trips the CE.
So, I always go to the other modules first....cuz I know when the car is running like crap, the DM is finally going to trip the CE... but by then, there are usally so many codes being passed , that it can get confusing .
I try to get to the faulty codes from the running module systems as quick as I can to get around to some kind of sensible code registration....
By the time the CE pops , the determining faults are hiden in a mess of faults , each caused down the line by the previous one .
A simple vac leak can pop 1/2 a doz. codes by the time it gets to the DM