Failed Smog Check w/ high NOx, Why?

Question

My '93 Nissan Sentra 1.6 failed smog check. Here's my readings.

@15 mph

CO2 = 14.9, O2 = 0, HC = 31, CO = .04, NO = 770 (max = 720)

@25 mph

CO2 = 14.9, O2 = .1, HC = 16, CO2 = .02, NO = 773 (max = 774)

PVC = Pass

New sparks and rotor and caps were just installed. I did however run long before changing oil.

Last smog check I initially failed the test. I had the CAT changed and then my readings completely changed with a failed Nitrious Oxide but all other readings improved. Last time I ran a full tank with "Guarantee Pass" additive and took the test again and BARELY passed.

Should I do that again? Should I use higher Octaine Gas? (Been using regular)

Answer 1

does the car have an EGR valve if that is not working properly it will raise NOX, the car temp can effect NOX if it runs on the hot side. you can also have the CAT tested to see if it is working properly. Is the timming on you can retard timming a little that may lower the NOX just enough.

Answer 2

when ever doing emissions test always run your car on the best octane!
Let your tank go as dry as you can and then fill-it-up full with premium gas just for the emissions and run it on premium for a few days before the test to clear out the system! Also check your tire pressures adding about 4 psi gives you a better reading too less rolling resistance!

Answer 3

sounds like the egr valve is not redirected exhaust gases through the combustion chamber causing nitrates of oxygen(nox) to flow out of exhaust causing vehicle to fail emissions check egr system..

Answer 4

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Answer 5

YEA try puting in racing ffuel. cheak airfilter. change oil and u should pass