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Catalytic converters have been around for a long time. They first introduced converters in the mid-seventies, on American vehicles. The catalytic converter may go unnoticed, until it fails. Replacement of most catalytic converters will be very expensive. Fortunately, most failures are also preventable.

Why does a catalytic converter cost so much?

A catalytic converter has no moving parts. They consist of a metal shell and a ceramic honeycomb through which exhaust gasses pass. The honeycomb provides support for the catalyst. The catalysts are the heart of the converter. Catalysts in modern catalytic converters contain noble metals. They normally use platinum, palladium and/or rhodium. Because of the scarcity of these metals, they are very expensive. An original equipment catalytic converter requires a high content of these metals. This is why a quality replacement converter is so expensive.

What does a catalytic converter do?

A catalytic converter has no moving parts

The catalytic converter changes toxic chemicals in exhaust to less toxic gas. Untreated exhaust gas is largely nitrogen oxides, carbon monoxide and unburned hydrocarbon. The catalyst in the converter helps convert these into much less toxic carbon dioxide, nitrogen and water. It does this by chemical reaction and a great deal of heat.

Heat generated by a catalytic converter, changes the chemical properties of the gas passing through it. This heat can also rise to the point of destroying the converter.

What causes catalytic converters to fail?

Still servicable catalytic converter, internal view

Unburned fuel or contaminants entering the converter, can cause the heat to rise out of control. These contaminants enter through the exhaust. Excessive heat can melt the honeycomb support.

Badly overheated and plugged catalytic converter 

An overheated honeycomb may break and cause a rattle. It can also block the exhaust flow. This results in the exhaust being restricted. Restricted exhausts result in a loss of power, fuel mileage, overheating and usually a check engine light.


Common causes of contaminants entering the exhaust

Coolant, from leaking intake-manifold gaskets, is often drawn into the engine. A leaking head gasket has much the same effect. The chemicals in the engine coolant quickly contaminate the catalyst and cause failure. Promptly addressing any loss of coolant helps prevent this expensive problem.

Using the wrong viscosity of oil may also damage the catalytic converter. Heavier viscosity oil may produce more vapors. These vapors enter the intake through the PCV system. Partially-burned contaminants pass into the catalytic converter with the other exhaust. Unburned hydrocarbons greatly raise the converter temperature.

A bad oxygen sensor, dirty mass air flow sensor or an engine misfire will also allow unburned fuel to pass into the exhaust. This fuel burns in the catalytic converter.

Oil from stuck piston rings, bad valve guides, etc. also enters the exhaust. Oil consumption often results from extended oil changes. This reduces engine efficiency and the oil passing into the exhaust damages the catalytic converter.

Too many short trips at low speed can also cause the catalytic converter to fail. Converters need highway driving to reach operating temperature. This is how they clean and regenerate the catalyst. If we do not drive our vehicle often or far enough, the converter may fail. Vehicles that get only intermittent use commonly have this problem.

Symptoms of catalytic converter problems

Converters fail in several ways so symptoms vary. Sometimes the ceramic breaks and causes a rattling noise. We most often hear this with the engine at idle speed.

An impact to the exhaust can cause the brittle ceramic to fracture. This sometimes occurs in a collision, when we hit the exhaust pipe. Foreign objects on the road can strike the converter. Improper service technique, such as pounding on the exhaust, may also cause damage.

Driving through high water is another common cause for failure. A catalytic converter gets very hot in operation. Quenching in cold water, damages the internal components of the hot converter. Rapid contraction, from improper cooling, may fracture the ceramic honeycomb in the catalytic converter.

Typical heated and non-heated oxygen sensors

The most common failure is the converter losing efficiency. When this occurs, the check engine light will come on. The power control module or PCM checks efficiency of the catalytic converter as we drive. A sensor submits a reading of oxygen content in the exhaust, before and after the catalytic converter.

Sensors monitor oxygen content before and after the converter

By comparing the oxygen content, before and after the converter, the PCM determines how efficient the catalyst is. When the converter no longer does its job, the PCM will turn on the check engine light to alert the driver.

A rotten-egg type odor, coming from the exhaust, may sometimes show when a converter is contaminated.

Toyota with three catalytic converters, one is out of sight 

When catalytic converters fail, we have to replace them. Some can be quite expensive. Many vehicles use two, three and even four converters and cost can run well more than $2000.00 for replacement.

An early warning system

Ignoring a check engine light may damage the catalytic converter

Protecting the catalytic converter is largely the job of the oxygen sensors and the check engine light. Oxygen sensors help keep the fuel/air mixture in the engine correct. This helps to keep excess fuel from damaging the converter. Oxygen sensors also monitor the efficiency of the converter(s). These sensors come in several types. Most modern engines have oxygen sensors that contain heaters. This helps them get to work faster.

An engine misfire may be the fastest cause of problems for the catalytic converter. Engineers provide the check engine light with monitors to check for misfires. They program the PCM to flash the check engine light when they occur.

Ignoring a flashing check engine light is an invitation to disaster. We must address a flashing check engine light immediately to avoid converter problems. The cause A misfire can destroy a catalytic converter in a matter of minutes.

The EPA mandated catalytic converter warranty

The EPA mandates a warranty on most new vehicle converters. This warranty covers the converter and installation. Catalytic converters, found defective, may receive a no-charge replacement, for the term of the warranty. The term of the warranty is normally eight years or 80,000 miles. A few vehicles may also have a warranty extension beyond this. Always check with a reputable shop for advice on warranty replacement if your vehicle has less than eighty-thousand miles and is less than eight years old.

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