I should have also mentioned a very simple test,, sorry,, but tap both Cats with a plastic or rubber mallet,, if you hear ANY rattles inside the Cat it's BAD.Ok, the 1st Cat. or Pre-Cat. should be at or near 550 degrees externally(internally it's much much higher than that close to 1500 degrees), this MUST be at that temperature because the Oxygen release/storage wont take place below that temp. The 2nd Cat is just a monitoring device that checks the 1st cats efficiency, this is ALL it does. It also gets very hot in operation, almost 550 degrees externally. As I said, the 1st Cat's Oxygen sensor will show a voltage from 200 Millivolts to 850 Millivolts and switch this voltage rapidly. The 2nd Cat or Post Cat will show a steady voltage around 450 Millivolts and hold this voltage with LITTLE variation. If the Post cat varies a lot in voltage or spikes in voltage then the cat needs replaced. Cats are 'killed' by other problems, usually misfires, overly rich conditions such as burned or bad sparkplug wires, bad injectors, injector wiring, coolant in the comb. chamber due to intake gasket failure, bad sparkplug. A bad O2 sensor wont cause a backfire,, you can even run a vehicle without O2 sensors, it's fuel economy will drop about 10-15%,, many many people drive around with bad sensors and dont even know it,, especially when the vehicle is older. Post yer voltage findings and remember to use a sharp needle at the wiring harness plug near the sensor,(Back probing) ,,if you can get to it easy. If not prick the wire,, then after testing wrap the area where you poked it with electrical tape!! (Seriously)