Your not ready to blow your free re-test yet, in my opinion. I would do the what Rob suggests, except after checking all vacuum lines, throttle body and switch, I'd check the mixture. To check, hook up a digital voltmeter to the output of the O2 sensor, ground the neg lead, once warmed up the reading should fluctuate from ~0.3 - 0.6 volts (300 to 600 mV). There is a good chance your O2 sensor is dead, especially if it's a single wire, they get real sluggish after ~40Kmiles. Also, when you adjust mixture by turning screw under plug on AMM, if nothing happens, you AMM idle circuit is out (get a rebuilt). Cat is last to replace. Good luck.