2000. Code 0172. Do smell gas from exhaust. This happened all of a sudden. Cleaned MAF and replaced cam sensor (had spare). Misses while driving.

 

This reply is a bit late, but I wanted to post for future readers. P0172 is an overly rich condition. Although you are headed in the right direction, 1999-2000s had coil packs that are prone to failure (when compared to other Miatas). They are a little costly, but the general fix is to replace the two small packs that the spark plugs pop onto.

It doesn't take a genius to replace, but it does require some small hands and a few colorful words to:
1. Disconnect a tiny 10mm bolt at the bottom of the Coil Pack bracket.
2. Disconnect the electrical harness at the coil packs. The bummer is that it gets hot back there and the plastic gets brittle over time, so be patient even though your hands are probably faigued.

If the coilpacks fail, rich conditions should be fixed asap. The car will still drive, bit you're drowning the oxygen sensors and catalytic converter in fuel, which will cause them to fail and cost more money.