title: Finding vehicle fuel-air equivalence ratio with OBD
body: <p>I'm trying to write an app that communicates with the OBD port in a car and (among other things) find the AFR, but my car doesn't seem to support the fuel-air equivalence ratio command <code>01 44</code>. I see that there are two sets of <a href="https://en.wikipedia.org/wiki/OBD-II_PIDs#Mode_01" rel="nofollow noreferrer">PIDs</a> for the oxygen sensors that give a fuel-air equivalence ratio. The first set is PIDs <code>01 24</code> through <code>01 2B</code>, which give</p>
  <blockquote>
  <p>Oxygen Sensor [1-8]</p>
  <p>AB: Fuel–Air Equivalence Ratio</p>
  <p>CD: Voltage</p>
  </blockquote>
  <p>The second set is PIDs <code>01 34</code> through <code>01 3B</code>. These look similar, but instead give the following data:</p>
  <blockquote>
  <p>Oxygen Sensor [1-8]</p>
  <p>AB: Fuel–Air Equivalence Ratio</p>
  <p>CD: Current</p>
  </blockquote>
  <p>Is the fuel-air ratio the same value on all of the sensors (at least ideally), or are they designed to measure different values? Which PID(s) should I use to calculate the AFR or FAR, and what calculations are required apart from what I see in the <a href="https://en.wikipedia.org/wiki/OBD-II_PIDs#Mode_01" rel="nofollow noreferrer">link</a> (which is <code>2/65536 * (256A + B)</code> for the ratio)? What are the voltage and current values, and are they of any use to me in this situation?</p>
  <p>Any help is much appreciated.</p>