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Simple Diagnostic Testing 



The conversion process produces heat. So, exhaust gases entering the converter should be cooler than the gases exiting the converter. The temperature differential test measures the surface temperature at the inlet and outlet bushings of the converter. If conversion is taking place, then the outlet bushing reading should be higher than the inlet bushing reading. However, different pipe wall thicknesses and corrosion, along with different heat transfer rates may cause inaccurate results. So the EPA recommends that this test be used only to prove that a catalyst is good.

Make sure that the engine is fully warmed up and running and that the heater is off. Using a pyrometer or infrared thermometer, read and note the temperature of the pipe just ahead of the converter inlet at the weld ring. The weld ring is the point where the inlet pipe is welded to the converter body. Read and note the temperature at the weld ring of the outlet pipe. Calculate the difference between your readings. If the outlet reading is higher than the inlet reading, you can be assured that at least some conversion is taking place. On well-tuned, newer vehicles, the catalyst can be fully functional at only a 20°F difference. 


This test measures the levels of HC, CO, CO2, NOx, and O2 gases coming out of the tail pipe. Using this data, you can isolate problems under the hood and under the vehicle.  
Excessively lean or rich mixtures are beyond the capability of the catalyst to convert. Even if the converter is working to specifications, measurable levels of pollution will be noted at the tail pipe under these conditions. The validity of this test to isolate a converter problem depends on a properly tuned and operating engine. Fortunately, a gas analyzer can first be used to locate engine problems.  
Following the manufacturer's instructions, connect analyzer to exhaust pipe and then read and note the levels of oxygen, carbon dioxide, hydrocarbons, NOx, and carbon monoxide produced by the vehicle. Compare your data with the 5-Gas Diagnostic Chart. The chart will help isolate engine emissions control problems. There may be more than one engine-related problem, in addition to a failed converter.  

Testing for an Exhaust System Restriction

These tests are used to determine if blockages exist in the exhaust system, creating excessive backpressure. The following tests can help determine the location of an exhaust restriction. 

Engine Vacuum Test 

The intent of the vacuum test is to determine if there is a blockage or restriction in the entire exhaust system. It may or may not indicate converter problems. This test can be performed using any suitable engine vacuum source. After the vacuum gauge is connected to a vacuum source, readings are noted at idle and then again at 2,500RPM. 

As engine speeds vary, readings should initially drop slightly, then rise to within 2-3" of the vacuum level established at idle. A large drop of 8-10" of vacuum typically indicates an exhaust restriction. Erratic swings of the vacuum readings may indicate periodic blockages caused by loose components temporarily blocking the exhaust system. Remember that vacuum levels are also affected by factors other than exhaust system restriction including valve and ignition timing. 

Backpressure Testing 

If a vehicle fails the engine backpressure vacuum test, you can pinpoint the component in the exhaust system causing the concern by measuring backpressure at different points in the exhaust system. These measurements can typically be made through the O2 sensor ports. Start by inspecting the system for crushed, bent or otherwise restricted exhaust pipes. Replace or repair as required. 

If the system passes the visual inspection, remove the most rearward O2 sensor. This sensor is typically located directly after the converter. The use of heat or penetrating oil and a back and forth motion may help if the sensor threads are rusty. Most O2 sensors use an 18mm-threaded port, so one adapter fits many applications. Install the adapter in the O2 port and tighten to manufacturer specifications. Then connect the gauge hose to the adapter. Begin by reading and taking note of the pressure at idle and at 2500RPM. On most vehicles, idle back pressure should not be higher than 1PSI. At 2500RPM, the reading should not be higher than 3PSI. 

If the readings are high at this point, the blockage probably exists downstream of the test point, which typically means it is in the muffler or resonator. If the back pressure is okay at this location, move to the O2 sensor just ahead of the converter and retest. If the back pressure is high at this point, the converter is causing the issue. If the O2 sensor ahead of the converter tests okay, then the restriction is most likely in the y-pipe assembly, or there could be an internal restriction in the air gap pipe located between the exhaust manifold and the converter.

Typical gas analyzer readings
Readings that would indicate a properly
functioning engine and converter (at idle)  


14.5 - 16%


0 - .35%


.1 - .45%


0 - 35 ppm


.995 - 1.005

readings that would indicate a properly functioning engine, but a converter that is not lighting off, due to contamination or physical damage


13.5 - 14.5%


.3 - .7%


.5 - .9%


75 - 125 ppm


.995 - 1.005

Please note converters will not light off unless Lambda is between .98 - 1.02

Gas The converter works best wih
co2 High co2 readings
  • co2 is an indicator of complete combustion
  • Higher readings indicate high efficiency
  • Single cylinder misfire in a 4-cylinder engine reduces reading by 25%
O2 <.5% Balanced with <.5 CO, but normally not zero
  • 02 reading should be close to but often slightly lower than CO reading (well within .1% of CO) 
  • High readings indicate a lean condition that can result in a false P0420 code
co <.5% Balanced with <.5 O2, but normally not zero
  • CO reading should be close to but slightly higher than the O2 reading (well within .1% of O2)
  • High readings indicate rich condition and are an indicator of incomplete combustion
hc <35ppm
  • High HC reading can result in excessive carbon restricting the converter
  • If high HC readings exist along with O2 readings, a misfire or cylinder imbalance may be inferred - This may cause the converter to overheat and the shell to glow red and discolor, resulting in substrate meltdown
  • Lower HC readings are always better for engine and converter efficiency
The lower the better
  • Slightly lean conditions are the main reason for high NOx readings
  • Exhaust leak prevents converter from reducing NOx efficiently
  • NOx is normally tested on a dyno with engine at partial load
lambda Should always be 1 or extremely close
  • Lambda is a more accurate indicator than AFR because it is actual balance of air to fuel
  • Converter requires Lambda to be between .98 and 1.02 to light off but will not work at peak efficiency unless Lambda is between 0.995-1.005
afr 14.7 Parts air to 1 part fuel or gasoline
  • 14.6-14.8 Is acceptable as long as Lambda is 1
  • Lambda is a better measurement because it indicates true air to fuel balance
  • It isn’t a theoretical target like AFR of 14.7:1


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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