hi @scottykilmer, 
I have first time smog check trouble for my 1998 Nissan 240sx in CA (milage 170K, automatic, no drifting, 99% stock - no changes), never had such situation before, always was passing, was kind of relaxed regarding smog, but these days I got stuck - I just before the check I changed oil as usual, warmed up car, went to the station and failed ASM emission HC test on 25 mph (HC max=42, HC ave=6, HC meas=54), all other ASM params for 25 mph look OK for me:
RPM=1738, %CO2=15.5, %O2=0, CO% max=0.5, CO% ave=0.03, CO% meas=0.1, NO max=738, NO ave=62, NO meas=38 (might be not perfect but OK for an old car). and, engine light is OFF, no codes - ECU not complaining. also should say that car stayed in garage since last good smog check (2 years, with old oil, with old gas, and battery disconnected, no gas stabilizer).
went to auto mechanic for diagnostics, explained the situation - my main concern was about 2 years in garage - I suspected that gas degraded (I never used stabilizer), carbon built up, and HC elevated - so I assume we see that failure (I'm not a pro, I just apply my logic).
BTW, before previous smog check (2 years ago) car also stayed in garage - I put it to garage at covid times because of remote work from home (before covid it was my main ride), and before  previous smog check I didn't even notice how ECU set all monitors readiness - I just warmed up car before the smog check and all drive cycles were set like a magic. this time I spent some time on EVAP monitor (all other monitors were just fine right after 10 miles ride after I put battery) - for EVAP I had to find a place to make non-brake deceleration from 55 to 0 - after I found the right place (some rural road with 55 limit) it took 3 min ride on 55 mph and non-brake stopping (I did it twice and it was done).
I'm giving all the details to show you that the car is more or less in good shape - I do not ride it as a crazy and I think it's not ready to be disposed yet.
so, with all that said, mechanic did his job:
- scanned computer - no pcm trouble codes present
- checked for intake and exhaust leaks - none found
- input readings in lambda calculator - fuel trim within spec
- checked ignition - incorrect spark plugs (OEM but incorrect heat range) - replaced with correct
- rechecked emission readings - still HC elevated
- performed cranking test on CC (catalytic converter) - that showed that CC is weak
and after that cranking test his verdict - CC must be replaced (and huge bill for that)
I tried to resist but he insisted that he bet the CC is weak and with the new one it will pass.  
BTW, I had no idea that cranking test is so important for Cat.  I'd expect HC level the highest on cranking because Cat is cold - but no complaints - he might be right, he has experience.
Now I still have time till the end of the month to correct the issue (or put car on PNO).
By my logic, if I suspect CC's fault then I'd check: temperature diff (before and after CC), back pressure of exhaust (to understand if CC clogged - but I asked mechanic about car acceleration  responsiveness, and he confirmed that it runs well - that wouldn't be if CC clogged).
I'm not confident because of lack of experience in this field but I still think that the the problem  related HC buildup in fuel system. Mechanic is still waiting for my decision ("you will have money from tax return, so use it here"). And I'm trying to understand what could help me to reduce HC buildup and hopefully emission, and pass the smog test. I'm considering fuel additives just to try to pass the test, I want to try to pass without too much expenses.
So, question is - which additive most likely could help in such situation (I understand that nobody guarantees, it's OK). which approach to consider?
- ATS 505 CRF (didn't see reports of 505 helping to pass smog but think it has potential)
- CRC GTP (? do not know but saw reports - it won't touch CC)
- "lacquer thinner" (this might go to CC but it could take time to clean the CC)
- ethanol (might be refuel in Costco, or pour ethanol into fuel tank - it will reduce % of HC)
- SeaFoam (not sure if that helps with CC or smog)
I need suggestion from experienced pro, I do not want to set extreme experiments for the car which is still OK. I'm curious, what would you do in such situation? (I do not like PNO idea)  
sorry for the long message,
thanks