Here's a few links to the Ford Service Manuals.
This is Section 4, it lists possible causes for every code a car code throw.
http://iihs.net/fsm/?dir=506&viewfil...scriptions.pdf
Once you look through Section 4, you can find the different "pinpoint tests" for the 3.7 Mustang here.
http://iihs.net/fsm/?dir=508
Looking really quick here, for p0019 you would go to pinpoint test HK. For some reason the 3.7 pinpoint tests won't load on my laptop, maybe they will work on your end. Hopefully this will help.