TunedRX300

fuel filter on the RX is supposed to be lifetime and not easily accessable.
I passed California smog test but also noticed above average HC. instead of mph I was tested with two rpms:

idle measured at 39, ave is 17, max allowed is 100
2500 measured at 15, ave is 13, max allowed is 170

but my CO% is 0.07% and 0.05%

High HC is an indicator of unburned fuel, perhaps using lower octane gas might help since 87 is more volatile than 91. You may try to clean the intake manifold, carbon buildup may occur over time. But I would check the cat first, it may be dying.

Lexmex

The 50 down to 8 is normal compared to what happened on your first test when it went up. The only thing that comes to mind was something in the fuel or it is not getting enough air at low mph during testing, whereas on the road you would be passing 15 mph due to a slight more amount of air (a miniscule ram air effect) as opposed to just he vacuum even with a stock system.

For those of you who haven't cut up your air box and modified your air intake system, the air that goes into the engine comes from a point just behind the headlight, that air comes from a narrow sleave nearby that takes air from the area where the air resonator sits (or where my bullhorn assembly sits, if I set up a fan in front 15 mph ala dyno, you would pass under this scenario. I'll take a picture later to show where this.)

One way this scenario could occur would be based on the temperature of the air going in being too hot and having a slightly hard time than colder air (since oxygen is more compressed)getting passed the air filter, like sitting in idle on a hot day in line for testing. That does happen with cars here in Mexico City with the long lines at the testing system. I wouldn't take out one's air filter as that can lead to too much air and the sensors can detect this, too. A non-oiled K&N might do the trick.

Lexmex

I found pictures in my gallery.

I wanted to add that when it goes up to 25 mph, there is a lot more backpressure than 15 mph in the exhaust, plus that vacuum of air is going to be stronger and like it or not, that hotter air will get passed the filter a lot easier 25 mph than 15 mph.

Where the arrow is, shows the top side of where the majority of the fresher air comes in from as opposed to the engine bay itself.

Lexmex

And from the underside. Note the ray of light above the point of the arrow.