There is a test you can do on the secondary O2 sensor (the one on the Cat) It requires removing the sensor. You can mount the sensor in a vise, and hook up the wires to a digital multimeter. Put the meter on DC volts 20V scale and hook the green wire on the sensor to the black wire on the meter. Hook the white wire on the sensor to the red wire on the meter. Use a propane bottle torch to heat the end of the sensor until it glows red, the meter should indicate between 0.4 and 1 volt. The voltage should drop to 0.1 volt or less in less than 4 seconds when you remove the torch. You can also test the sensor heater element with the meter by hooking the meter wires up to the two black wires on the sensor. It doesn't matter what order. Set up the meter to read resistance (ohms) on the 200 ohm scale. The sensor should read between 5 and 10 ohms when cold. Resistance goes up when the sensor is hot. If you don't get any reading, the heater circuit is blown, if you get 1 ohm or less, the heater is shorted. Don't touch the green or white wires with the meter leads when the meter is set up for resistance, the meter induces a small current in resistance mode and could damage the O2 sensor.