Toyota Repair: Toyota T100 Emissions, air fuel mixture, toyota t100

Question
I have a 6 cylinder manual transmission 1995 T100. Mostly 20 miles Back and forth to work, mostly rush hour freeway driving. I have been passing emissions for the past 6 years with a bad rear O2 sensor. This year I failed NOX with a 3.01, Phoenix AZ limit is 3.0. HC & CO passed with .17 & 5.43. Toyota replaced both O2 sensors (codes) and plugs and plug wires. Retest resulted in NOX failure of 3.59 and HC & CO increased to 1.3 and 9.16. I checked for any lines that may have come loose ( while in the shop I had a P/S hose replaced) and sprayed around to check for leaks. Engine temp guage reads normal, coolant level is OK, cooling system was flushed 3 years ago. It's going back to toyota next week. Any Ideas?

Answer
Higher than normal CO readings indicate a rich air/fuel mixture, higher than normal NOx is caused by excessive combustion chamber temperatures, this is why EGR systems were added solely for the purpose of redirecting some of the exhaust gases back into the intake system, the high nox may be caused by a malfunctioning egr system or higher than normal combustion chamber pressures, this can be caused by excessive carbon build up in the chambers, it can also be caused by incorrect ignition timing.