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nox is increased when:
cylinder temperatures are too high (carbon, motor running hot)
ignition timing is too advanced (base timing or timing advance)
defective egr (not just the valve, the vacuum sources clogged, etc)
lean fuel mixture (low fuel pressure, vacuum leaks, shitty injectors, bad fuel pressure regulator)
if all the above is perfect, check the catalytic convertor. (more than once before you get a new one, these shits are expensive, and dont go bad for no reason, no matter what the shady smog guy tells you)
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