93 Century Emission Questions

The car failed emissions on NOx, everything else within spec. Test is an acceleration on a dyno from 0 up to 30 mph and then done to 0. The test runs for 30 seconds. NOx spikes at 8 seconds to about 12 seconds and then
drops off to nothing by about 20 seconds. Normal, since that's the acceleration part of the test, but it exceeds the limit by about 1/3. Although the common cause is the EGR, the engine doesn't have one (even though the garage said they were getting me a price on changing it, after they told me the car failed).
I've run through the diagnostics and at this point, I'm at a loss. Two possible causes are low MAF readings and failure of the knock sensor. The MAF reads about 14 gpm at 2500 RPM (read live from the ECM) and the knock sensor shows 0 throughout acceleration. The MAF sensor wires have been cleaned, with no difference. I've also checked its passage. The knock sensor has been tested and it responds to raps on the block. I've also checked its connection. The oxygen sensor is fairly new and seems to respond properly.
Does anyone know where I can find typical operational values for these sensors? Or have any other suggestions? The engine is a 3.3L, with about 112K on it. It runs well, especially since I had the injectors cleaned. I put some Guaranteed to Pass in, hoping that the problem is carbon build-up, but I'd feel better knowing that everything else is in shape. I could just change the MAF and the knock, but at $210, I don't want to just waste the money.
Thanks for any help.
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It's not unlikely that the catalytic converter is worn out. The reduction part of the converter that removes NOX is often the first to go.
High NOX could also be caused by a non-working EGR (but this has none), incorrect spark timing (unlikely,.assuming this is a DIS system) or overly lean fuel mixture (not likely, if the O2 sensor is responding normally and switching at a sufficiently fast rate).
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I've considered the cat, but I keep reading about mechanics trying to solve problems with high NOx by putting in a new one and finding that it didn't make any difference. Again, I'm a bit leery of the time, effort and expense of swapping out a part if there isn't much too chance that it will fix the problem. I've heard that the cat only has minimal effect on NOx, but this is the first mention I've heard made that the NOx treating part of the converter often fails first. I'll hold that as an option, then.
Thanks for the input.

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That is a thought, although I've kept an eye on the engine temp (OEM guage) and it seems to be in the normal range. However, I've noticed that I've been getting some higher temps in the oil recently (from an extra temp guage in a triple). Definitely worth checking into.
Thanks for the input.
And it *does* sound profound.
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