Auto Systems and Repair: Catalytic Converter Replacement
The catalytic converter looks similar to a muffler. It is typically a stainless steel housing with a ceramic honeycomb inside that is impregnated with platinum, palladium, and rhodium. These elements help clean up harmful exhaust gases that otherwise would be expelled from the tail pipe.
Mechanics Corner: More Technical Detail
The catalytic converter is a sophisticated after-burning device designed to complete combustion of the exhaust gases that pass through it. It is a stainless steel container with an inlet and outlet pipe that looks similar to a muffler. Inside, the catalytic converter is a ceramic monolithic structure that has honeycomblike passages running though it. This structure has several sections called beds that are thinly coated with rare metals that react with the compounds in the exhaust gases to complete the combustion process, thereby cleaning the exhaust of the harmful emissions.
- The first section of the catalytic converter is called the reduction bed and is coated with rhodium. It is called the reduction bed because its purpose is to reduce the NOx gases back in to harmless nitrogen and oxygen.
- The next section of the catalytic converter is the oxygen storage bed, which is coated with cerium. Its purpose is to maintain an ideal level of oxygen for use by the rear of the converter. It does this by storing and releasing the oxygen that gets released from the reduction of NOx in the previous reduction bed.
- The oxygen is then available for use in the final oxidizing bed, which is coated with platinum and paladium. The purpose of the oxidizing bed is to complete the combustion of CO by adding oxygen. The oxidizing bed also uses oxygen to burn any of the raw HCs that still remain in the exhaust gases.
The inside of the catalytic converter operates at very high temperatures. It will not even begin to light off until it reaches 400-600 degrees Fahrenheit; it typically runs from 1200-1600 degrees depending upon how hard it is being worked. A rich- or dirty-running engine can make the catalytic converter work so hard that it will actually glow red. This is why there are catalytic converter–damaging misfire codes that illuminate the check engine light—and even make it steadily flash under severe misfire conditions.
A failing catalytic converter has many implications.
- It can cause the vehicle to fail an emissions test.
- It can set the check engine light.
- The insides can break apart and become very noisy, making a rattling sound that can become unbearable.
- It can be contaminated by coolant, oil, or fuel and produce an offensive smell.
- The inside can get too hot (from a poorly running engine) and melt. This restricts the gas flow, which causes the vehicle to lose some power, all of its power, or to not run at all.
- Catalytic converters can be vehicle specific, especially in the high-performance lines. In many cases, however, a new OBDI (pre-1996) or OBDII (1996 and later) catalytic converter made by a quality manufacturer that meets or exceed the emissions requirements—and costs much less than the factory unit—can be welded into place.
- It is important to have a qualified emissions diagnostician inspect the vehicle to determine if and why the catalytic converter failed. In some cases, a catalytic converter becomes less and less efficient as the mileage increases—usually over 150,000 miles. But in many cases, a catalytic converter is ruined by an improperly running engine. This root cause needs to be resolved or the new catalytic converter will fail in a short time.