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Dealing with AFS

Understanding the operation and troubleshooting of air/fuel ratio sensors
Wednesday, August 5, 2015 - 07:00
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A simple solution
If you put me on the spot and asked me what the stoich voltage value was for each manufacturer I would probably have a hard time remembering them all. I try not to remember anything that I can look up. However, the real reason I have a hard time remembering is because I don’t use voltage or amperage for diagnosis. I use lambda. Lambda is such a perfect solution that manufacturers such as Toyota and Honda provide it as a PID. They even go as far as teaching their techs to use lambda instead of voltage or amperage. It’s really simple once you get the hang of it.

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Lambda is a fantastic tool for diagnosing AFS and related complaints. Lambda is a ratio of available oxygen compared to the combustion processes’ demand for oxygen. Lambda is equal to 1.000 when there is balance between available oxygen and demand for oxygen. In other words, lambda is equal to 1.000 when the current air fuel ratio matches the desired air fuel ratio.

This formula is known as the Brettschneider formula, named for Robert Brettschneider who first proposed it in 1979 in a technical paper published by Bosch. While his original formula is quite complex, it is extremely useful in determining air/fuel ratios and imbalances. iATN.net provides a lambda calculator on its website. You can access it at MotorAge.com/lambda.

The lambda calculator will allow you to enter values from an exhaust gas analyzer to determine lambda. Why is this important? Well, if you had a misreporting sensor, there is really no better way than using the gas analysis and plugging the numbers into the calculator. While this doesn’t happen all that frequently on the Asian products, it is a good idea to keep this method in your back pocket for future reference. By the way, don’t get rid of that old gas analyzer!

Should you suspect that you have a sensor that is not reporting properly the best method is to plug your five gas values into the calculator. Once the numbers are plugged you can use your calculated lambda value to determine the actual running condition of the engine. For example, if your calculated lambda number is .997 It is pretty safe to assume that the engine is running at Stoich. If the calculated lambda value is less than or greater than 1.000 multiply the lambda value by 14.7. The product of your multiplication is the determined AF ratio. For example, if the lambda calculator value is .887, multiply .887 x 14.7. Your resulting AF ratio is just about 13:1 which is slightly on the rich side. For another example, suppose that your lambda calculator returned a value of 1.250. Again, multiply 1.250 by 14.7. The air fuel ratio is roughly 18.4:1 (lean). Do this a few times and you will have the hang of it. Always remember a lambda value of less than one is a rich mixture while a value of greater than one is a lean mixture.

If you are having trouble trusting the calculator you can always use manual testing methods to drive the mixture lean or rich and plug the values into the calculator to see the change in lambda value.

While I prefer to use a scan tool to do so, you can substitute pulling off a vacuum hose to create a lean condition. To drive the mixture rich the use of propane is another option. The bottom line with both methods of testing is that you are looking for a near instantaneous change of the sensor values. While most newer vehicles will code if a sensor is lazy, manual inducing a rich or lean condition can provide some peace of mind that the sensor is reporting rapidly.

As with any diagnostic routine, the best way to learn is by doing. Try these simple formulas and test methods on known good vehicles before trying to fix a problem vehicle.

λ= Current A/F Ratio ÷ Stoich

Current A/F Ratio= λ × Stoich

To use lambda in diagnosis you can simply take the lambda value and multiply it by stoich (14.7).

Stoichiometric Ratio


3.3 Volts

0 mA


A Lambda calculator can help find a skewed AFS. (iATN.net) To use Lambda in diagnosis you can simply take the Lambda value and multiply it by stoich (14.7). A comparison of AFR, Voltage, Amperage, and Lambda values for Toyota vehicles.
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