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A catalytic converter is a mechanical device designed to convert the three most harmful elements present in a car’s exhaust into harmless elements. The three harmful elements are Hydrocarbons (unburned gasoline), Carbon Monoxide (accumulated by gasoline combustion), and Nitrogen Oxides (formed when the nitrogen in the air is forced to combine with oxygen by the heat in the engine). A catalytic converter aids in converting carbon monoxide into carbon dioxide. It also helps in converting the hydrocarbons into water (H2O) and carbon dioxide. The nitrogen oxides are converted back into oxygen and nitrogen. A catalytic converter has no moving parts and is designed to last the normal operating life of the vehicle.

Most catalytic converters fail due to engine-related problems. Replacing the catalytic converter without diagnosing and repairing the cause of the failure may lead to another ruined converter. Typically, converter failures fall in one of the following categories: physical damage due to corrosion or from the converter coming into contact with a large object on the road surface.

Potential Causes:

  • Overheated, melted, or broken converters
  • Misfires – Low compression, low spark, or no spark
  • Engine mechanical, ignition, or control system failure
  • Coated/Oil-Fouled Substrate
  • Excessive oil consumption (burning oil): bad rings or valve seals
  • Excessive carbon build-up in exhaust: Incorrect timing of fuel mixture, faulty spark plugs or plug wires, faulty check valve, oxygen sensors
  • Internal coolant leaks (head/intake gasket)
  • Improper fuels or additives: E85, diesel, sulfur (found in some low quality gasoline)
  • Structural damage
  • Physical damage such as dents or cracks from road debris, collision, speed bumps, etc.
  • Metal fatigue or stress fractures
  • Corrosion
  • Contamination due to excessive oil consumption, internal coolant leak, or excessive carbon build up
  • Melted substrate due to engine misfires which lead to excessive converter temperatures
  • Thermal Shock or Cold-quenching: hot converter is suddenly cold quenched when driving through deep water or into deep snow
  • Sudden drop in temperature forces the converter housing to contract, which can cause cracks or breakage of the ceramic substrate.
  • Converter aging/lack of engine maintenance – cycles of damaging engine conditions will eventually deteriorate converter performance
Converters will get red hot when excess fuel is introduced directly into it, along with sufficient oxygen to burn the fuel. This is not a problem with the converter itself, but the result of a problem with the fuel system or ignition that allows unburned fuel to pass through the engine and then travel down into the converter. If the root cause is not corrected, the new converter will melt as well. Common causes of a melted converter are: 1) A three-way plus air vehicle running rich, and when the air is injected into the converter, the rear brick will melt as the excessive fuel now has enough oxygen to burn inside the converter, 2) Vehicle is running rich with an exhaust leak, and when the air is drawn into the exhaust pipe and is combined with the excess fuel, it will burn in the converter, 3) The vehicle has a misfire. When the air-fuel charge leaves the combustion chamber without firing, it will travel through the exhaust pipe and burn in the converter.
If a converter is operated too long at a high temperature, the substrate may “melt down” and turn into a solid mass inside the converter. The vehicle may seem sluggish as if there was a loss of power. Other causes might be: 1) upstream converter has broken up and the debris has clogged a downstream unit, 2) the support mat may have become damaged and no longer retaining the brick in the correct position, allowing the brick to shift and block the exhaust flow.
If a replacement converter fails after a short period of time, then the root cause of the original failure has not been addressed. Some causes are contamination by silicone-based sealants, coolant leaks, oil blow by, high sulfur fuel, and/or rich fuel mixtures forming carbon deposits can quickly coat the substrate preventing it from working effectively.
No. Converter meltdown is primarily contributed to an engine misfire. The unused air and fuel resulting from an engine misfire will cause an intense fire inside the catalytic converter, damaging it internally. Normal operating temperatures of a converter are 500-800°F, and up to 1200°F when the vehicle is under heavy load. To melt the catalytic converter’s substrate, the temperature inside the converter would have to exceed 2000°F.
A P0420 low efficiency code does not always indicate that the converter needs to be replaced. On newer vehicles, a low efficiency code can occur if the exhaust feed gases are not of proper balance to allow the converter to operate efficiently. An experienced emissions technician may be able to identify and resolve this concern with a scan too. The most effective way for most technicians to diagnose this condition is through the use of a 5 gas analyzer, performing the switch-ratio scan tool test, and Oxygen Storage Capacity (OSC) scan tool test.


It is illegal to install a catalytic converter based solely on physical shape, size, configuration, or pipe diameter.


  • Determine Vehicle Manufacturer
  • Determine Vehicle Model
  • Determine Vehicle Year
  • Determine Vehicle Specific Engine Size
  • Vehicle Test Group / Engine Family Number
  • Visit or the California Air Resources Board Aftermarket Converter Database to see which aftermarket part is approved to install.
  • If no EXACT match is found, then there are no aftermarket catalytic converters available at this time.
When installing a universal converter, it must still meet the emission requirements of the vehicle and cannot be chosen by size alone. By looking up your specific vehicle in the catalog or web site, you will find the recommended universal converter with the appropriate loading. If a universal converter is not listed for your specific application, a direct fit must be used; if a direct fit is not listed, the only alternative is the Original Equipment Manufacturer (OEM) part.
No. A diesel engine has different emission requirements and a gas engine converter will not function on a diesel engine.
The best way to determine this is to look at the vehicle’s emissions system label. The label can usually be found on either the front radiator support, the strut tower plate, or under the engine hood. If the vehicle is California Emissions Certified, the label will reference “CARB,” “California,” or “ARB.” Once the emissions certification has been found, the “Engine Family Number,” or sometimes referred as “Test Group Name,” “Engine Family Code,” or “Group Number” must be determined. This number can be found on the Emissions Control Information Label.

Every CARB-compliant replacement converter must display a certification stamp or label on the converter shell that includes:

  • CARB Executive Order approval number
  • Manufacturer Part Number
  • Date of Manufacture
  • Exhaust Flow Direction


Engine changes present a problem and challenge to car owners and technicians. Contact your local Bureau of Automotive Repair (BAR) to make sure your vehicle is compliant to the standards.
Yes, if it has been tested for this type catalyst by the manufacturer in compliance with CARB aftermarket converter procedures.
There is no guarantee that replacing the converter will keep a fault code from coming back. If engine performance issues exist, and it has not been repaired, the P0420/P0430 fault code may reoccur.

Yes, since converters are designed to last the life of the vehicle, the technician should identify and correct the root cause of the original converter failure.


  • Make sure any other codes are corrected prior to installing the new converter. This is especially true for misfire, mass air flow, rich/lean conditions, and O2 response rate codes.
  • Pressure check the cooling system to test for leaks which could contaminate the new converter.
  • Repair any exhaust leak that may be present. A stethoscope or a smoke test are successful ways in detecting an exhaust leak. However, a smoke test can not only find escaping exhaust, but also air that is being sucked in. An exhaust leak may affect converter and O2 sensor operation.
  • Check O2 operation - The front sensor should have good frequency, amplitude, and response rate and average 450mv. The rear should be fairly steady at idle and above 450mv (typically 650-850mv).
  • If both of the above O2 sensor readings are not present, the vehicle should be checked with a 4 or 5 gas analyzer and repairs should be performed.
  1. Complete a warranty card in triplicate with the original going to the customer, one copy to the installer, and one copy to the manufacturer of the converter.
  2. Retain a copy of the warranty card for a minimum of four years from the date of the installation.