Free ASE L1 (Advance Engine Performance) Certification: Lambda
BAR Diagnostic Specific Course
This Free ASE Prep online study guide is also from my smog license repair course
Lambda is used to describe the air/fuel ratio; a value of “1” equals a balance air/fuel mixture 14.7:1 (14.7 pounds of air to 1 pound of fuel)
Values over 1.020 will indicate a lean air/fuel mixture as were a value under “1” (0.980) will indicate a rich condition.
This diagnostic reading should be part of your emission troubleshooting but you will need a calculator.
Google Lambda calculator or if you are a member of iatn.net there is one there on the site.
The proper Air/Fuel range is 0.980 to 1.020 will allow the most efficient converter operation.
These readings can be taken pre-or post converter gases, the converter will have no effect on the these readings.
Now this range is efficient to oxidizing HC and CO it’s not good for Nox, the reduction in Nox will be better control when the Air/Fuel is closer to 0.995.
The converter is efficient in Oxidizing HC and Co on the lean side of the air/fuel chart. Nox is better controlled on the rich side of the air/fuel range.
So a lambda reading less than 1 (<1) would be a rich condition, in most cases Nox will be low.
Example: if you have a vehicle that fails smog for Nox and the vehicle has a Back pressure EGR and a lambda reading less than <1, the vehicle did not fail for a lean condition.
The vehicle could have failed for:
Low exhaust gas back pressure
Excessive carbon deposits in the combustion chamber
Old freeze plugs causing the vehicle to run hot
A vehicle fails the smog check for high Nox, when you put emission readings into the calculator, the results are as shown.
Air / Fuel Ratio (A/F) Ratio 15.89
Do you think this vehicle fail for high Nox because of the EGR, Timing or something else?
Yes! This vehicle fails because of a lean condition and not the EGR or Timing problem.
Remember the maximum reading is 1.020 anything over is a lean condition which is good for Oxidation for HC and Co but to lean Nox will increase.
The next reading example is another vehicle that fails the smog test, so we input the readings into the calculator and the results are a perfect “1”.
So this vehicle didn’t fail for air/fuel supply problem so we have to check the ignition timing, EGR or cooling system.
The next example is a vehicle that failed Nox with high O2 and a Air/Fuel reading of 1.021. In this case the fuel supply and the emission components check out good so now it’s time to check the catalytic converter.
When using the lambda calculator to evaluate the air/fuel mixture, the exhaust must be leak free and the Air injection system disconnected. The exhaust gas sample can be taken before or after the catalytic converter, on a hot or cold engine.
If there is an exhaust leak in front of the O2 sensor the PCM will sense a lean condition from the O2 sensor and the command from the on board computer will be to increase fuel delivery, and now a rich condition will result.
If the exhaust leak is in front of the catalytic converter the Rhodium section of the catalytic converter will not reduce Nox, remember the front section needs carbon monoxide not o2 to reduce Nox.
Now if you have an exhaust leak behind the front or upstream O2 sensor and in front of the catalytic the vehicle can have a false catalytic converter code.
Also if the exhaust leak is between the catalytic converter and the back or downstream O2 sensor again a false converter code could set.
Remember before replacing a catalytic converter check the air/fuel readings with the Google calculator and confirm there is no exhaust leaks before checking the system.
NOTE: If the engine is operating at a lambda value of 0.880 (rich condition) this would compromise the life of a catalytic converter