What causes high HC and CO during smog check?

my car failed with both twice as high as the max allowed. i read that it could be because of a rich fuel condition but what can i do to fix it? new o2 sensor? Before i took it to smog i had changed the oil, spark plugs, spark plug wires, pcv valve, and poured into the gas tank a fuel injector cleaner. The car also seemed to be running good. What could cause the problem? its a 97 camry 2.2 liter