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Diagnosing Catalytic
Converter Issues


Catalytic converters can only function efficiently if the engine is operating properly and the exhaust system is leak-free. Diagnose the cause of emissions problems with 13 key steps.  

Obtain vehicle history

Customer feedback is an invaluable tool. Drivability, performance and service history can help guide you through the diagnostic process. Ask the customer the following questions:

  • Has the catalytic converter ever been replaced?
  • What service has been performed in the last 6 to 12 months?
  • Does the vehicle require that coolant or oil be added regularly?
  • Has the fuel economy decreased recently?
  • Is the engine hard to start, either hot or cold?
  • Does the engine run smooth?
  • Have you noticed any engine misfires?
  • Has the vehicle run out of fuel?
  • Did the wrong fuel get added? 
  • Do you use fuel additives?
  • What is the condition of the coolant?
  • Does the heater work properly?
  • Have aftermarket parts been added?
  • Performance PCM programmer (chip)
  • Performance exhaust
  • Remote starter
  • How is the vehicle used and driven?
  • Towing with transmission in low gear
  • Parking brake engaged, putting extra load on engine
  • Locked in 4WD low for extended periods of time

Any condition that increases emissions or affects sensor readings could cause a converter to fail diagnostics even if the converter is good. (Many OBDII trouble codes will affect converter performance.) Correct all other codes prior to correcting converter codes.

The following codes are related to the catalytic converter:

  • P0420 Catalyst system efficiency below threshold (Bank 1)
  • P0421 Warm-up catalyst efficiency below threshold (Bank 1)
  • P0422 Main catalyst efficiency below threshold (Bank 1)
  • P0423 Heated catalyst efficiency below threshold (Bank 1)
  • P0424 Heated catalyst temperature below threshold (Bank 1)
  • P0430 Catalyst system efficiency below threshold (Bank 2)
  • P0431 Warm-up catalyst efficiency below threshold (Bank 2)
  • P0432 Main catalyst efficiency below threshold (Bank 2)
  • P0433 Heated catalyst efficiency below threshold (Bank 2)
  • P0434 Heated catalyst temperature below threshold (Bank 2)

Leaks in the exhaust system can affect O2 (oxygen) storage in the converter and lead to improper O2 sensor readings, affecting the AFR (air/fuel ratio) balance. Do the following:

  • Check all weld areas for cracks, especially O2 sensor ports.

  • Examine all pipe connections for improper alignment or burnt gaskets.

  • Check all clamp connections for leaks.

  • Pay close attention to any flex-pipe in the system.

Using an infrared thermometer, check the temperature of the converter's front and rear weld rings, to ensure the converter has "lit off." Depending on their sizes, most converters begin to light off around 350°F, and are fully lit around 500°F. 

Under normal conditions, the rear weld ring may reach temperatures which are as much as 150°F higher than the front weld ring. If the rear weld ring reaches temperatures in excess of 150°F higher than the front weld ring, the engine may have an emissions problem. 

  • Keep in mind that the converter's rear weld ring temperature is directly related to the amount of work the converter is performing. Therefore, elevated temperatures may indicate an emissions issue.

  • If the rear weld ring is significantly cooler than the front, the converter may not be lighting off. This may indicate the converter has failed, or that the exhaust mixture is not correct, which is a symptom of an underlying emissions issue. 

  • Typically, converter temperatures will not exceed 1200°F on a properly running engine. Periodic operation above 1600°F can negatively affect the precious metals coating on the substrate, reducing its efficiency. Excessive temperatures can reduce the converter's durability, or, if high enough, destroy the converter's matting or substrate.

  • Damaged matting and melted substrates typically occur at temperatures exceeding 1700°F. It is possible to test for a cracked substrate or damaged matting by tapping on the converter housing. Using a rubber mallet, "thump" the shell, listening for loose components.

  • A bronze/blue rainbow discoloration of the shell typically indicates elevated temperatures. If the converter is removed, look through the substrate to observe whether the small passageways are melted or collapsed. The substrate may actually appear normal at either end, since the substrate melts internally.

Excessive system backpressure can generate a converter efficiency code. Although backpressure varies with application, typically the pressure should be less than 3 PSI at 2,000 RPM and less than 1.5 PSI at idle. 

  • Using the O2 sensor ports, check for excessive backpressure ahead of - and behind - the converter
  • High backpressure behind the converter indicates a restricted muffler or resonator
  • High backpressure ahead of the converter indicates a restricted converter
  • High backpressure at the exhaust manifold indicates a blocked Y-connection
  • Excessive backpressure will adversely affect AFR, leading to excessive emissions

O2 (oxygen) sensors are critical to fuel control and to the PCM's converter diagnostics.

  • The front sensor should be very active and typically oscillate rapidly from approximately 0 to less than 1 volt. If the front sensor shows low or no voltage, the sensor could be defective; or, there might be an exhaust leak before or immediately behind the sensor.

  • Typically, the rear sensor should emit a fairly steady signal. If the signal is below 250 mv, check for activity by rapid accelerator kickdown, or by raising the engine speed to approximately 2,000 RPM. Some movement should be noted.

  • Check O2 sensor operation to ensure vehicle is in fuel control. On most applications, the front O2 Sensor should be switching around 450 mv and the rear O2 sensor should be above 450 mv, typically 650 - 850 mv. If the rear O2 sensor is not above 450 mv, check the vehicle's emissions with a 4- or 5-gas analyzer. The front AFR sensors should indicate AFR of 14.7:1 under most conditions.

  • Many diagnostic tools exist to accurately test O2 sensors. Verify operation before replacing any sensors.

Internal cooling system leaks can destroy a catalytic converter.

  • An internal leak in the cooling system would allow coolant to enter the exhaust system and poison the catalytic converter.

  • Perform a leak-down test to make sure the cooling system holds system pressure (check pressure cap for exact pressure) for 15 minutes.

  • If the cooling system is not maintained properly, it may build up sludge, which may reduce heat transfer from the combustion chambers, increasing NOx emissions.

There are many things in the fuel system that can damage a catalytic converter or increase HC (Hydrocarbon) emissions.

  • Perform a fuel pressure and leak-down test. Fuel pressure should be within the manufacturer's specifications and hold steady after the pump is shut off.

  • A rapid drop in pressure may indicate that fuel injectors are leaking or that a problem exists with the fuel pressure regulator or the supply check valve.

  • Fuel pressure which is slightly shy of the normal working range can result in significant emissions issues, and can also result in excessive cranking time when cold. Both conditions can cause rapid converter failure.

  • The fuel filter should be replaced per the manufacturer's recommended maintenance schedule. Finally, check for water contamination in the fuel tank and air trapped in the fuel rails which can cause cylinder misfires.

The PCM attempts to maintain AFR of 14.7:1, using data it receives from the vehicle's sensors, primarily the MAF and O2 sensors.

  • Short-term fuel trim is based on readings from the current engine operating conditions. Long-term fuel trim averages several of the short-term readings to affect the fuel base mapping.

  • Any fuel trim reading greater than 5% indicates a problem that requires checking cylinder balance, and/or checking with a 5-gas analyzer.

  • Remember, fuel trim is just one of several systems that affect emissions. The fuel trim may be correct, yet the engine may still be producing excessive emissions.

One of the most effective ways to resolve emissions issues is by sampling the exhaust gases. Keep the following in mind:

  • High HC emissions indicate unburned fuel.

  • High CO levels indicate partially burnt fuel, or oil.

  • High NOx levels are normally caused by high combustion temperatures and pressures, slightly lean AFR, and excessively advanced ignition timing.

  • Tailpipe emissions readings low in HC and CO levels with high NOx emissions are typically NOT caused by a defective converter. The low HC and CO readings indicate that the converter is functioning. The root cause of the problem is an engine which is emitting excessively high NOx emissions. These high NOx emissions may reduce the durability and efficiency of the converter.


One of the most effective ways to resolve emissions issues is by sampling the exhaust gases. Learn more about gas analyzer readings by viewing the 5-GAS DIAGNOSTIC CHART.

Any cylinder that is not working as hard as the others is the most likely cause of emissions issues.

  • Most professional scan tools can accurately perform a cylinder balance test by dropping one cylinder at a time.

  • With an infrared thermometer, look for cylinders that are running hotter or colder than others. This can indicate a lean-or-rich condition. Likewise, it can indicate high-or-low cylinder compression.

  • Rear cylinders which are running progressively hotter than the front indicate cooling system deposits (rust or sludge) restricting flow to the rear of the engine block. This can result in high NOx emissions due to increased combustion temperatures, even though the average coolant temperature reads normal.

High-mileage engines can have many mechanical problems that contribute to high emissions.

  • If the vehicle is so equipped, the EGR valve and passages should be inspected for proper operation, and cleaned, if necessary.

  • Perform a compression check to identify the condition of the piston rings, valve train and combustion chambers.

  • Check the camshaft lobes and the timing belt or chain for wear. Verify cam and ignition timing.

Re-flashing is the process of updating the PCM (Powertrain Control Module) with the latest program available from the vehicle's manufacturer.

  • In some cases, manufacturers have released Technical Service Bulletins indicating that a re-flash of the PCM will help resolve emissions problems.

  • When a vehicle is new, there may be "unseen variations" that occur with the aging of the engine components.

  • PCM updates are usually very specific to the vehicle build dates and options.

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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