Understanding OBD-II 02 Sensor/Fuel Trim results

[simplecomtools]

We are looking for help understanding the meanings behind the results from PIDs related to 02 sensors and Long and Short Term fuel trims (LTFT and STFT).

We are working on an OBD interface that will poll the ECM for data on user configurable time intervals for remote access/monitoring.

We can pull the 02 sensor and fuel trim data - but do not really understand their inter-relationships or what the values means in the real world. Here are examples of the data we are extracting...

PID - Description
1B - OBD2.STFT24
1D - OBD2.02SEN2
1F - OBD2.LTFT01
06 - OBD2.STFT01
07 - OBD2.LTFT01
etc.....

We are looking for ideas on what resources are available to understand what these values mean and what presumptions should be made based upon the data being provided by the ECM. We are telemetry specialists - not trained mechanics - and are reaching out for ideas/suggestions from the automotive community.

Thanks in advance for your time....
SimpleComTools

 

[MikeHalloran]

Basically, an O2 sensor's output is digital; it goes up and down as the engine control bounces around the setpoint. Let me rephrase that last a bit; the O2 sensor output changes state at a particular a/f ratio. The engine can run at a setpoint other than that, by biasing the control such that the sensor has an asymmetric duty cycle. The control also infers stuff based on the frequency at which the sensor rattles around, and the way the frequency and the duty cycle change with time and with temperature.

You probably knew that, and more. To get it in words that mechanics use, you need to go to Pep Boys and buy the books the mechanics buy. It might be helpful to buy a variety of books from different publishers, covering different carlines and different systems, so you get an appreciation for the variations in the jargon involved.

The jargon has nominally been standardized, by SAE, which also standardized what appears on the data bus and in what form. Everything you could possibly want to know is at sae.org, somewhere, in some Jxxxx standard or other, all of which are for sale. Be prepared to browse a while, or call them and ask for help, or buy one of their collections. The amount of information available is staggering; the _catalog_ doesn't fit on one CD.

You might as well join, so you can get a modest discount on the big handbook and on the technical papers and CDs and books you will want.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA