Toyota Repair: 1990 toyota corolla w/ high NOX, toyota corolla, 4afe

Question
My 1990 Corolla did not pass DEQ because the my NOX (3.39g/mi) exceded the Oregon limit of 3.0g/mi.  It passed the rest of the emisssions tests.  
It has a 4AFE EFI federal version engine w/ no ERG valve and ERG modulator.
Can you give me some suggestions on causes and how to fix this high NOX problem.  

Answer
With an egr valve the engine adds exaust to the combution chamber to cool the burn. Even though the exhaust is hot, its cool compaired to the combustion temp.
Cars without one use the o2 sensor and the ecu changes the fuel injection duration.
I would start with an o2 sensor because its so close.
It may end up needing a Cat converter.
Change the oil and make sure the engine temp is normal.


Mike