Toyota Repair: Cause of high nitrous emissions, combustion temperatures, nox emissions

Question
I have a 1993 Toyota Camry 2.2 litre which failed the State of Ohio "E check" testing program. Nitrous emissions are set @ 910.0 ppm and my vehicle failed with elevated emissions of 2024.1 ppm. Also, the HC and CO emissions are elevated, but not above the limits. I have had several conflicting theories as to what may be causing this problem including a bad catalytic converter or faulty oxygen sensor or bad EGR valve or timing off, or c combination of any or all of the above depending on which person I talk to. The car has 134,000 miles on it, has been regularly maintained by the previous owner, and runs like a scalded dog when you tromp on it (as best as a 4 cyl can). I'm stumped and need to pass this test to get the car licensed. Please reply ASAP as the temporary tags are ready to expire "again". Thanks! Tony  

Answer
I assume that you mean the NOx reading is too high, the most common cause of this is that the combustion temperatures are too high for one reason or another as I will soon explain, one of the reasons the EGR system was added to engines is to reduce NOx emissions by recirculating some of the exhaust gases back into the intake system reducing the combustion temperatures.
If there is a malfunction with the EGR system it can cause the cumbustion temperature to increase, it can also be caused by incorrect timing of the ignition system, too much advance, or cam timing. Normally, the egr failure will cause the check engine light to come on but not always on the early OBDI system, I recommend having the egr system tested to see if it is operating normally.
Since the HC and CO readings are somewhat normal, at least below the limits, I can assume that the engine does not have a misfire or fuel management problem, but since I don't have the actual readings I don't know for sure if that is the case.
Make sure that the engine is not overheating, if everything checks out ok, the catalityc converter may be the cause, the oxygen sensors won't cause high NOx readings.
It is possible to test the catalityc converter by connection a diagnostic scantool and reading the #1 and #2 O2 sensor waveform voltages, the technician should be able to compare the front and rear O2 sensor readings, if both readings are the same the catalityc converter is failing.
I hope this explains your question to some extend and let me know if I can be of more help.