It looks like you are just slightly above the limit at 15 mph, 875 to 760. It must not be necessary to pass the 25 mpg limit for NO b/c you didn’t pass it in 2010 but you still passed the smog test then. So focus on the 15 mph NO.
I think both the N and O are coming from the intake air. Air is 70% nitrogen, 20% oxygen. Under high heat the two must can combine and form NO. NO is the chemical nitric oxide, and is an air pollutant. Seems like you have from among three problems
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Carbon build up – this stuff forms hot spots which are considerably hotter than the rest of the cylinder and piston surfaces, so it could form NO in those spots.
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EGR malfunction. The EGR is supposed to open enough to cool the cylinders for the main purpose of preventing nitrogen oxides from forming, like NO. If the EGR isn’t working properly, this could result.
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Catalytic converter is on the fritz. The cat is sort of the back-up system to remove any remaining nitrogen oxides that come out the exhaust manifold and turn them into (I guess) N2 and O2 where they go out the tailpipe to live life anew. No harm, no foul.
So what would I do in this situation? hmmm … well, first off I’d want to make sure the ignition system is working 100%, so I’d install a new distributor cap, ignition rotor and spark plug wires, assuming that is how yours is configured, an electronic ignition using a mechanical distributor to move the high voltage around. I’d make sure the battery and alternator are in good condition, and I’d charge up the battery with a battery charger just before the test. A 10% decrease in voltage to the ignition system could result in a 20% less energetic jolt at the spark plug. I doubt that will solve this problem, but then you’ll be starting with a known good ignition system.
Next I’d test the EGR to make sure it is working at least minimally. On my Corolla, what I do is hook up a vacuum pump to it, apply some vacuum, and it should stall the engine at idle. Even if it does, I’d still remove it and take a closer look. Any signs of carbon deposits there, I’d replace it. And I’d clean out all the EGR ports – the holes through which the exhaust gas recirculates into the intake manifold.
Next I’d make sure the vacuum system that controls the EGR is working. That varies a lot from car to car so you’d have to look at your service manual. My Corolla has three tiny vacuum ports at various points in the throttle body which feed the EGR system, and the computer gets involved too. There’s also an EGR temperature sensor on the Corolla, so if you have one too, make sure that is working.
Finally I’d do what I could to insure the mixture isn’t too much on the lean side. Lean mixtures result in higher combustion temperatures. So check for small vacuum leaks, maybe remove & do a clean-up of the inside of the throttle body.
To remove unnecessary complications that might have unintended consequences, I’d revert to plain dino 10-30 oil, and a copper ngk plug, maybe the v-groove version for a little better igniting spark.
If all that doesn’t do the job, next up would be a new cat.
Those carbon deposits – if you actually have them enough to be causing this problem, which I’m dubious – are super hard and practically impossible to remove without disassembling the engine. I wouldn’t worry much about those. Best of luck.
Edit: I’m presuming the engine air filter is in good shape.
