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From the articles at
http://www.team-integra.net/
WHY DID IT FAIL?
1. Out Of Tune Engine
Anytime an engine is running out of tune due to improper air/fuel mixture, misfiring cylinders, faulty engine sensors, incorrect ignition timing, etc., damage to the catalytic converter will be incurred. Proper and regular servicing per the auto manufacturers recommendations are necessary to prevent premature catalytic converter failure.
2. Excess Fuel Overheating The Catalytic Converter
An engine that is performing at peak efficiency will burn all the fuel in the combustion chamber during the combustion process. An engine that is not performing properly, that is not burning all the fuel, will allow unburned or excess fuel to enter the exhaust system. When this excess or unburned fuel contacts the hot core of the converter it will ignite. This constant infusion of unburned fuel will cause temperatures to continuously rise above the designed operating temperature until the core of the catalytic converter will actually melt. Possible causes for the excess fuel entering the exhaust system are an incorrect fuel mixture, incorrect timing, corroded spark plugs, worn and cracked ignition wires, improper fuel pressure, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.
3. Oil or Antifreeze Entering Exhaust
When oil or antifreeze enters the exhaust system and contacts the hot core of the converter the oil and antifreeze will burn off leaving carbon deposits. The carbon deposits will coat the core of the converter thus reducing the catalytic converter's ability to convert from harmful emissions into harmless compounds. As the carbon deposits continue to accumulate, the pores in the ceramic catalyst will become restricted and block exhaust flow through the exhaust system. The resulting increased backpressure will result is a loss of power and overheated engine components. Possible causes are worn piston rings, faulty valve seals or valve guides, blown head gasket or intake gaskets, or warped engine components.
4. Malfunctioning Oxygen Sensor
The oxygen sensor measures the amount of oxygen present in the exhaust gas. Depending on the voltage generated by the oxygen sensor, the engine management system will change the air/fuel ratio to obtain the desired oxygen level present in the exhaust gas. A malfunctioning oxygen sensor sending an erroneous reading to the engine control system can cause a too rich or too lean condition. A rich condition will cause the converter to overheat and melt down from the unburned fuel being ignited while a lean condition can result in a misfire that can lead to the same result. Oxygen sensors wear out and need to be changed per your auto manufacturers time and mileage limits.
5. Broken Exhaust Hangers Or Misaligned Exhaust
An exhaust system that is misaligned or allowed to rattle will cause the fragile ceramic catalyst inside the converter to break apart. When the core breaks or becomes loose in the converter the brittle ceramic catalyst will continue to break up into smaller pieces that will eventually block the flow of the exhaust. This increased backpressure will lead to loss of power and heat build up
