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5-Gas Diagnostic Chart

Monitoring Exhaust Gases

Of the many molecules and compounds coming out of the tailpipe, the EPA is primarily concerned with five of them:

  • Carbon dioxide (CO2)

  • Oxygen (O2)

  • Carbon monoxide (CO)

  • Hydrocarbons (HC)

  • Oxides of nitrogen (NOx)

One of the most effective ways to resolve emissions issues is by sampling the exhaust gases:

  • 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

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

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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