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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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Factory tooling

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Toyota factory tooling is the least expensive factory tool on the market. A Techstream Lite Mongoose cable from Drew Technologies retails for around $500. Techstream software can be purchased through subscription at www.techinfo.toyota.com.

The Techstream provides a great active test for the AF sensor. This test provides the abilities of manually creating rich or lean conditions by changing the fuel injection volume or pulse width. With the scan tool you can now achieve the same desired conditions from the driver’s seat of the car. The active test allows you to toggle the air fuel ratios instantly rich or lean while graphing sensor response. This is my go-to test when I suspect a lazy sensor. Many aftermarket scan tools provide similar functionality.

To scope or not to scope?
Using a digital storage oscilloscope for AFS is not a realistic or worthwhile venture. Remember that the AF sensor outputs a current flow, and as such we would have to use an inductive current clamp to graph the value on the scope. Being that the sensor generates just milliamps, the use of a highly accurate low current clamp at a cost of over 700.00 needs to be used. My advice: take the $700 and take your significant other on a weekend getaway.

When it comes to diagnosing AF sensors it is important to remember that the manufacturer has gone to great lengths to build diagnostics into their ECM. Diagnosis should be done primarily with a scan tool. In the case of Honda and Toyota, these sensors are relatively reliable. Many of the early problem sensors resulted in published Technical Service Bulletins, and using TSBs as a first step in diagnosis is a best practice when fixing Asian vehicle fuel control faults. Always remember the K.I.S.S. method. Diagnostic routines shouldn’t be complicated. Use the info provided here to work smarter and not harder. 

Most 2003 and new Toyota products use active AF control to test the AF and O2 sensors as well as catalyst efficiency. Drew Technologies Mongoose cable provides full factory functionality for Toyota Lexus Scion. Techstream active tests for AFS provide a great way of testing AFS response.


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