Because you seem to consistantly bring up the AIR fault code along with 19, I would give a fast check to the AIR system, specially the vac lines.
You mentioned that you have checked the main engine vac feed
line to the SOV that I mentioned in an earlier post, so I would next check the one that goes from the AIR SOV to the AIR Control Valve. This is the valve between the Pump and the exhaust ports 3 and 4 in the head that, when vac from the energized SOV reaches, allows the pumped air to get to the CAT. [for rapid cat heat and hc emmissions aid]
If this line is leaking/off the Air control valve, the AIR system will not work and a code will pop. At the same time , the line can cause a lean condtion.. enough to do a 19, ???? prob not ... but worth a look see.
The reason I mentioned the EGR and Purge valve leaks before is that these are both large lines and could pop a 19. [ too much for the stop limits to correct]
If these lines/ valves seem to be OK, check and see if pump is getting power, etc. for that code..