I. IS IT DEFECTIVE ?
Fig. Top is a NORMAL CAT, Middle is a Failing Cat, Bottom is a Defective cat:
Catalytic converters are defective when they become clogged or poisoned. You may sometimes be able to feel when a converter is partially clogged, or defective, when you do not go any faster when you step on the throttle. This may also be accompanied with a noticeable drop in gas mileage. A clogged converter will cause increased backpressure in the exhaust system. This increased backpressure prevents the engine from breathing properly which in turn may cause the engine to quit after a few minutes of driving or feel like an e governor limiting the RPM's the engine can achieve. You may actually hear a whistling or choking sound when applying the throttle.
A catalyst that has broken apart internally and is loose in the system or has worked its way back to the muffler can also cause rattling noises. This rattling may be prevalent at idle but go away as speed is increased. Sometimes you may tap on the converter with a rubber mallet and hear the core rattling. If you discover any of the above your converter needs to be replaced.
Since there is no inspection port for you to see if you have an actual clog in a converter there a couple things you may try to help diagnose a defective converter. Many mechanics will remove the oxygen sensor and look for a change in the vehicles performance. If a performance change is noted, your converter needs to be replaced. You must be extremely careful if you run the engine with the oxygen sensor removed since there is a risk of fire and asphyxiation caused by escaping engine exhaust and exhaust gases.
A vacuum gauge that also has a fuel pressure gauge, or a pressure gauge that has a 1 to 10 pound scale, may also assist you in diagnosing a clogged exhaust.
If you connect the gauge upstream of the converter, either in an emission test port or by fashioning an adapter in the oxygen sensor hole, you can measure the exhaust backpressure. A general rule of thumb is no more than 1.5 pounds of backpressure. Another test, using a vacuum gauge, is to measure the engine vacuum at curb idle and at 1600 RPM. If engine vacuum is 21 inches at curb idle and 15 inches at 1600 RPM then there is a good possibility your catalytic converter needs to be replaced.
You may also take the converter off the vehicle and check it for damage or clogging. Shining a flashlight into the core of a honeycomb style converter will reveal whether the honeycomb is partially clogged, damaged, or loose from the edges.
Finally, a converter can be defective because it has become poisoned. When a catalytic converter becomes poisoned it loses its ability to effectively control emissions. Some common causes for poisoning and converter degradation are using leaded fuel, fuel additives containing lead, an out of tune engine, engine management system failure, faulty engine sensors, just to name a few.
If your vehicle is burning an excessive amount of fuel or misfiring, due to some of the reasons above, it is just a matter of time before converter failure. A constant rotten egg smell emitting from the tailpipe is a sure sign of the impending failure of your converter.
If you do determine that your catalytic converter is defective, it is extremely important that you determine the reason for its demise. Always have your emissions checked on an exhaust gas analyzer after the new converter is installed to ensure your engine and engine management systems are operating at their peak efficiency. Failure to find and repair any fault will result in a premature failure of your new converter.
II. 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