What Type of Problem Do You Get From a Bad Catalytic Converter?

by Chris Stevenson
itstillruns article image
Thinkstock/Comstock/Getty Images

The catalytic converter in your car serves as a kind of an afterburner, using heat and special chemicals inside a metal case to burn off harmful exhaust gases before they exit the tailpipe. The underlying problems associated with catalytic failure stem from other components and systems that do function properly. Catalytic converter problems will show up in various ways, and the car driver or owner should be aware of any precursor signs before irreversible converter failure.

Basic Catalytic Converter Construction and Design

Basic catalytic converters consist of an outer steel casing filled with a substrate of chemical materials. Two types of chemical materials exist: BB-sized ceramic pellets and monolithic honeycomb plates. The coating on the pellets or plates consists of precious metals such as palladium, platinum and rhodium. The front or header side compartment holds the reduction catalyst, while the rear compartment houses the oxidizing catalyst. The converter sits just behind the main header pipe, which exits the exhaust manifold. It is typically welded or clamp-bolted on the header pipe.

Oxidizing, Two-Way Catalytic Converter Function

Oxidizing, or two-way, converters are more simple in chemical design than three-way converters and minimize hydrocarbon and carbon monoxide pollutants. Oxygen, carbon monoxide and hydrocarbons enter into the converter, interacting with the substrate materials, which causes accelerated oxidation or a flameless burning. The increased oxidation renders the harmful emissions dead or chemically neutral, and they exit the exhaust as harmless carbon dioxide and water vapor.

Three-Way Catalytic Converter Function

The three-way catalytic converters functions similarly to the oxidizing converter, but it additionally reduce levels of oxides of nitrogen. For this purpose, these converters often have air injection tubes driven by air pumps, which accelerates the chemical transformation of the oxidation catalyst.

Overheated Appearance

The chemical substrate inside the catalytic converter can melt, super-heated by an overly rich air-fuel condition, spark plug miss or other ignition malfunction. Generally converters that have a 200 degree F difference or more between the inlet and the outlet side of the converter at normal cruising speed show indications of interior overheating and can visibly turn the outside shell casing black, with a rainbow- colored hue. An infrared pyrometer or contact temperature probe is used to determine if the converter is working too hard to expel excessive heat.


When the substrate within the converter melts either the platinum-palladium coating or the aluminum oxide honeycomb material, it forms an impenetrable slag, which blocks off the exhaust flow, clogging the passages. This creates a severe back pressure that affects engine performance. A clogged converter can cause rough idle, engine hesitation upon acceleration, weakened power, hard-starting and, in some cases, a no-start condition. Faulty components that cause overly rich fuel-air mixture, like cracked distributor caps, faulty EGR valves, incorrect timing or dirty fuel injectors can all cause melting. This symptom can be diagnosed by using a vacuum gauge on the intake manifold, or an exhaust back-pressure pressure gauge before and after the converter.


Fouling results inside the catalytic converter when excess unburned fuel is present. The core temperature of the converter case does not have to reach excessive temperature, such as a meltdown of the substrate, but higher than normal temperatures will result, as well as some increased back pressure. When the substrate chemicals are diluted, raw gas frequently exits the exhaust pipe in a black cloud. The substrate can actually become wet, or fuel-fouled. Gas mileage suffers, as well as acceleration and overall power.


Burned catalytic converters frequently give off a hot-metal smell, accompanied by the odor of burnt rotten eggs, which result from burning substrate and overheated metal. Some air pump rubber lines can melt, giving off a burnt rubber smell, as well as any fiber gasket or plastic component. Oxygen sensor wires can give off an acrid smell because of the burnt insulation.


For a catalytic converter to do its job properly, the ignition and fuel systems must perform to manufacturer's specification. This includes a properly tuned engine, with no misfires from ignition parts failures and improper adjustments to the timing and the primary and secondary ignition system. This checklist includes points, plugs, condenser, or distributorless ignition, coil, spark plugs, plug wires and all relative ECM sensors. The fuel system must have the proper pressure and adjustment on carburetor or fuel injected systems, where the air-fuel mixture is neither too lean or too rich. A properly tuned and timed engine will not damage the normal operation of a catalytic converter.

Fault Codes

Fault codes can appear on the dash that indicate a "Check Engine" or "Emission Control" problem. A scanner tool can record and reference a letter and number code, specifically outlining the affected part. For instance, a scanner that reads a code from P0420 to P0423 will indicate a catalytic converter that is not operating at peak efficiency. The converter may not be clogged with such a code, but it will show temperature and pressure differences at the inlet and outlet sides.


Check the warranty on your catalytic converter if you have a newer vehicle. Find out what parts come under coverage, along with the time period and-or mileage. Fill out the registration form and send it in. A catalytic converter warranty does not negate you from keeping your vehicle properly tuned to manufacturer's specifications. The time to catch a failing converter is during the very early stages, based on what part is causing the early failure mode signs.

More Articles

article divider