The front catalytic converter takes the bulk of emissions reduction load. If we had to take a guess (which you should never do when it comes to replacing a catalytic converter) we'd say the front CAT is defective. Only after all other emission systems have been inspected for proper operation should you suspect a defective or bad catalytic converter.
Since you need to check "one" specific catalytic converter it would be best to use the "intrusive" CAT Test method to determine exactly which cat, the front or rear, is damaged. Keep in mind the intrusive test only works if and when the rest of the vehicle's emission system is working properly. Fuel control must be accurate, the engine should not have compression or ignition trouble.
The Intrusive CAT Test requires an emission sample be taken prior to and after the CAT which is being tested. During the test there should be at least a 40% decrease in emissions after the CAT. A 5 Gas Analyzer will be required to perform this test; usually a smog machine (BAR-97).
A simpler test, but less accurate and one you can do at home using an infrared thermometer, is a Temperature Test. Using an infrared thermometer measure the exhaust temperature at the CAT's inlet, and then measure the temperature at the CAT's outlet. You should see at least a 600 degree temperature increase at the outlet. Perform this test only after your car has been properly warmed up and while the engine is at high idle (2500rpm). Be careful as the exhaust components will be very hot.
