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Home >   How To >   How to Repair Catalytic Converter >   Important Information about Catalytic Converters

Important Information about Catalytic Converters



A catalytic converter reduces the extent of harmful substances in the exhaust gases and releases them as a less-toxic gas into the atmosphere. There are no moving parts in the catalytic converter. The exhaust stream contains raw emissions such as hydrocarbons (HC), carbon monoxide (CO), and nitric oxide (NOX). The catalytic converter reduces the levels of these hazardous substances to acceptable standards required by the EPA (Environmental Protection Agency) and CARB (California Air Resources Board). The most common reason for replacing an OBDII converter is a PO420 code which indicates a “catalyst inefficiency”. This is caused when the rear oxygen sensor is reading a fluctuation that is greater than the vehicle computer's limits.


NOTE: The oxygen sensors only measure the oxygen retention of the catalyst, ignoring the levels of hydrocarbons, carbon monoxide and other toxic substances. *We have found that using a fuel that has a higher level of octane than the fuel recommended by the manufacturer can cause red light problems. It will also cause too much carbon build-up and trip the PO420 code.*


Causes of a Faulty Cat Converter


The following are constraints that include the major components to check before replacing a defective converter, and lists the common causes of a failed converter:


  • A malfunction in any of the emission-control equipment such as the oxygen sensor and engine coolant temperature sensor can prevent the converter from providing an optimal performance.
  • A vehicle that is not tuned on a regular basis can also cause the catalytic converter to go bad.
  • Problems with parts such as a carburetor, throttle body injectors or ported fuel injectors can induce problems in the converter.
  • Contaminations in the fuel system such as lead, oil, silicone, phosphorus, or coolant can also be detrimental to the catalytic converter.
  • Worn-out piston rings or valve seals also affect the performance of a converter.
  • Internal parts of the catalytic converter like the ceramic monolith can get damaged.
  • An engine that has a mechanical problem or is improperly tuned can cause a premature converter failure.
  • Over time, the cat converter can also become clogged with debris, requiring a flushing or cleaning.
  • ***CARBON BUILD-UP inside the engine is a major problem. This can cause the engine to run rich or lean, which in turn expedites the carbon build-up in the engine and other parts such as the catalytic converter and exhaust sensors. A regular inspection of the catalytic converter and flushing/cleaning it, if required, can prevent you from blowing a hole in your budget.


The manufacturer’s warranty will not cover converter failure that is caused due to engine malfunction or contaminated fuel.


Symptoms/Indications of a Bad Catalytic Converter


  • An obvious symptom of a bad catalytic converter is a high level of exhaust emissions.
  • Decreased engine power is also a symptom of a damaged cat converter. Reduced engine power in turn causes problems during accelerating, such as hesitating and bucking.
  • A reduced mileage and fuel economy can also be due to a damaged catalytic converter.
  • The engine may run at higher temperatures than usual despite the cooling system working at the optimal level.
  • A foul smell from the cat converter is another indication that the cat converter is deteriorating. This is due to the cat converter getting overheated, causing a high amount of hydrogen sulfide in the emissions.
  • A visible indication of a bad catalytic converter is black exhaust smoke. Though the black smoke can primarily be due to engine-related issues, it is also recommended to check the converter for faults.



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