I failed emissions today, what could be wrong?

I would soon suspect a failed O2 sensor before i would the converter! I would look at the O2 switching on a compatible scan tool to see if the PCM is in fuel control, The Jeep Cherokee O2 sensors, from the 1987 to 1990 4.0L engines, are a 5v sensor. They work backwards. The Powertrain Control Module (PCM) sends 5v to the sensor and the sensor resists it to ground based on the O2 content in the exhaust. The higher the voltage, the leaner the engine is running, the lower the richer. ADDED: @ old school how do you raise the idle on a IAC controlled vehicle? (only min.throttle angle is adjustable) (There is no MAF this is a speed density system) (If it had a vacuum leak the HC would be extremely high!!!) If your not knowledgeable in engine performance/emissions you shouldn't be answering this question! ADDED: Your leaning in the right direction Dinkles!