87 N/A won't smog, feedback?

The first time the car failed at 25 mph hydrocarbons, they said replace the cats ( I didn't). 2nd try at another place the car failed hydrocarbons at 15 Mph, this guy says it's not the cats since it was fine at the lower speed, and that the car may be running lean. What causes a car to run lean, and how do I remedy this? He suggested replacing the o2 sensor. I guess i'll do that. The plugs are fresh, wires are fresh, just changed the oil, timing and everything is good, good fresh gas, etc. What else should I consider/do?

Hmmmm..... Passing smog. . .

1. Move somewhere where there aren't smog regulations.

2. Bribery

3. Disconnecting your downpipe to let less through to the tip (That is where they measure it right?)

4. And if all else fails, try bribery again.