[Steve]

Plan ahead before you take it in for testing. This isn't a topic just for SSP Mustangs, or any particular internal combustion-powered vehicle. Any vehicle requiring periodic pollution control testing is affected by this. Most of you guys probably already know all this, but for others who don't know or like me didn't pay attention and think about it ahead of time, do so!
The problem is having them sit unused for long periods of time. I have other classic/unusual vehicles including a military vehicle (M1009 CUCV diesel) and am about 70% through an early 65 Mustang coupe restoration.
I also have a C4 Corvette, 1989 with a 350 and the 6-speed ZF manual transmission. It is a toy and I only drive it during good weather and when I feel like it. I live in California and 1976 and newer vehicles must be smog checked every 2 years if they are to remain registered. The Corvette is low mileage (68,000) and in excellent original condition. When I bought it it had no catalytic converters because the owner lived in a County that doesn't require ongoing testing, only when ownership is transferred, and he had removed them. I had new CATs installed and it tested very clean and far below even the average test results.
The registration came up for renewal with smog check required. I hadn't driven the car for well over a year. It had about a quarter tank of fuel and I had added the correct amount of Stabil to it so it would stay usable and it did. But I knew it would be good to fill it up with some fresh gas so I filled it up and drove 30 miles total. Figured that would be ok.
It wasn't. Everything tested well except for NOx, which was over the highest allowable amount by only 3 parts per million (ppm), but that was still a fail. I was annoyed because I knew the engine was in fine condition, and the CAT converters were still essentially new. Was it that since it had been sitting for so long the CATs needed to be run hot, burn off any carbon, reactivate? Was the Stabil an issue? Did some residue from it coat the CATs? Time and $120 wasted on that test...
So I drove the vehicle round trip to the SF Bay Area and around there some, putting about 450 miles on. Took it back for a retest and it tested very well, with the NOx output well below even the minimum level for the typical vehicles from that era.
So that was a lesson for me. If your vehicle has been unused for an extended time, especially if you added a fuel stabilizer, plan to run out that fuel, refill the tank at least once, then put enough miles on to restore/reactivate/burn off carbon, whatever is going on just get it done before you go in for the test.