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.