Converter Failure

What is a catalytic converter?

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 being Hydrocarbons (unburned gasoline), Carbon Monoxide (accumulated by gasoline combustion), 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.

Why did my catalytic converter fail?

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 to one of the following categories: Physical damage due to corrosion, or from the converter contacting a large object on the road surface.


  • Overheated, melted, or broken converters
  • Misfires – Low compression, low spark, or no spark
  • Typically caused by an 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 / Stress fractures
  • Corrosion
  • Thermal shock: Cracked substrate that could occur from driving through a deep puddle
  • 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.

What makes a converter become red hot and/or melt the brick (thermal melt down)?

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.

What has caused my converter to become plugged (loss of power)?

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.

What is chemical contamination of a converter (poisoning)?

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.

The internals of my catalytic converter have melted down after a few months of being installed on my vehicle. Was my converter faulty from the factory?

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.

Why is my check engine or MIL light indicating that I have a PO420: low converter efficiency?

A P0420 low efficiency code does not always indicate that the converter needs to be replaced. On newer vehicles a low efficiency code will can occur if the exhaust feed gases are not of the proper balanced to allow the converter to operate efficiently. An experienced emissions technician may be able to identify and resolve this concern with a scan tool, 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.


Find the Right Converter

Identifying the right aftermarket Catalytic Converter for my vehicle:

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


  1. Vehicle Manufacturer
  2. Vehicle Model
  3. Vehicle Year
  4. Vehicle Specific Engine Size
  5. Vehicle Test Group / Engine Family Number
  6. Visit or California Air Resources Board Aftermarket Converter Database to see which aftermarket part is approved to install.
  7. If no EXACT match is found, then there are no aftermarket catalytic converters available at this time.

EFN Diagram

How do you determine what universal converter to use?

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.

Can I use a standard universal type converter on a vehicle with a diesel engine?

No. A diesel engine has different emission requirements and a gas engine converter will not function on a diesel engine.

Is my vehicle manufactured for CARB or federal?

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.

How to identify a California Air Resource Board (CARB)-compliant catalytic converter?

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

  1. CARB Executive Order approval number
  2. Manufacturer Part Number
  3. Date of Manufacture
  4. Exhaust Flow Direction

Converter Installation

The vehicle has an engine that is older or newer than the vehicle or a larger engine has been installed. what type of converter, or does a converter need to be installed on this vehicle?

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. A general guideline can be found

Can I replace a two-way converter with a three-way converter?

Yes, provided it has been tested for this type catalyst by the manufacturer in compliance with CARB aftermarket converter procedures.

How do I determine if i have a California emissions vehicle?

To determine if a vehicle has California emissions, check the emissions sticker on the vehicle, which is located in the engine compartment. If the sticker states the vehicle meets California requirements, then the vehicle is equipped with California emissions. Click Here For Engine Family Number Information.

Will replacing my converter with a new one eliminate the PO420/PO430 code from coming on?

There is no guarantee that replacing the converter will keep a fault code from coming back. If an engine performance issues exist, and it has not been repaired, the P0420/P0430 fault code may recur.

Are there any steps my technician can take to prevent my new converter from failing prematurely?

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.– The technician should 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 checks the cooling system to test for leaks which will contaminate the new converter.– Repair any exhaust leak that may be present. A stethoscope or a smoke test is successful ways in detecting an exhaust leak. However a smoke test can not only find escaping exhaust, but 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.

As an installer of a new CARB Certified Aftermarket Catalytic Converter (AMCC), what are my responsibilities?

  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.