Toyota Repair: 1999 corolla fail emission test for high NOx, combustion chambers, engine cooling system

Question
I got 1999 corolla 150,000km, emission test failed for NOx too high (657ppm), the car has no EGR valve, no VVT-i. Emission test result is: at 40km/h HC 15ppm, CO 0.02%, NOx 657ppm.; At curb idle HC 21ppm, CO 0.00%.  
Two questions. 1.How the car get NOx controlled without EGR, by catalytic converter only? 2. How to pass the emission test on this car
Thanks a lot


Answer
With the HC and CO readings being below the limit you can rule out an engine mechanical problem, higher than normal NOx readings are caused by excessive combustion chamber temperatures, may be due to the engine running hotter than normal, the Nox is controlled by lower compression ratios and valve timing, the catalityc converter also recuces NOx gases, make sure the engine cooling system is good and the engine is running at normal temperature, there may be excessive carbon deposits in the combustion chamber which can raise the compression and cause this problem, there are chemical treatments available that will help decarbon the pistons and the combustion chambers and it may be worth a try to use that option.