Toyota Repair: 1998 4runner failed smog - conclusion, combustion temperatures, denso plugs

Question
I had a high NOX failure. Since I had no EGR system to fix I figured I would do everything that made sense including:

New fuel filter,
Ran a couple of tankfuls with injector cleaner,
Ran a bottle of Seafoam carbon cleaner (lots of smoke),
Took the new Bosch plugs out and put the old Denso plugs back in,
Replaced the front O2 sensor (probably unnecessary).

My NOX went from ~2500 to ~500.

I just hate not knowing why.

FYI: some local libraries now have online repair manuals - free to access.  

Answer
High NOx emissions are mostly caused by high combustion temperatures, this is why the EGR system was used and still is on some models, your engine doesn't have EGR because it was engineered to reduce combustion temperatures with other means.  The reason for higher combustion temperatures may be due to the engine running hotter than normal, excessive carbon build up in the combustion chamber(increasing pressure) incorrect ignition timing, running the carbon cleaner through was a step in the right direction but the other possible causes should also be addressed.