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Why a Catalytic Converter Fails

Diagnosing Converter Issues

Converters don’t fail on their own; failure is usually a symptom of a larger problem. This is why it is important to identify the root cause of the failure and perform the repair prior to installing a new converter. 

Most catalytic converter failures fall under one of three categories:

  • Overheated, melted or broken converters

  • Coated/oil-fouled substrate

  • Structural damage

Overheated, Melted or Broken Converters

Any malfunction causing an unusually high level of HC or CO (in conjunction with high levels of O2) to enter the converter, will dramatically elevate its temperature.  
Potential causes for high HC readings may include:

  • Misfires

  • Low compression

  • Poor spark or no spark

  • AFR cylinder imbalance

  • Excessive engine or vehicle load  
     - Fuel delivery system or fuel quality  
     - Sluggish (worn out) O2 sensors  
     - Excessive backpressure  
    NOTE: Temperatures in excess of 1600°F may damage catalyst, without visible signs of melting. 

Coated/Oil-Fouled Substrate

Catalyst poisoning occurs when the converter is exposed to emissions containing substances that coat the working surfaces, enveloping the catalyst to the point it cannot contact - and treat - the exhaust.  
Potential causes for coated or fouled substrate may include:

  • Excessive carbon buildup in exhaust

  • Internal coolant leaks (head/intake gasket)

  • Use of non-converter-safe gasket sealants

  • Excessive oil consumption (burning oil)

  • Improper fuels or additives (E85, diesel)

Structural Damage

The primary cause of structural damage is road debris striking the converter. Normally, some evidence of impact is visible on the converter shield.  
Other conditions that may potentially cause structural damage:

  • Corrosion

  • Thermal shock

  • Metal fatigue/stress fractures

  • Stripped O2 sensor threads

  • Flex pipe failure

  • Air-gap pipe failure

Learn more about quality exhaust parts, find the right car part, or find a local repair shop today.

The content contained in this article is for informational purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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