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 01 44. I see that there are two sets of PIDs for the oxygen sensors that give a fuel-air equivalence ratio. The first set is PIDs 01 24 through 01 2B, which give
Oxygen Sensor [1-8]
AB: Fuel–Air Equivalence Ratio
CD: Voltage
The second set is PIDs 01 34 through 01 3B. These look similar, but instead give the following data:
Oxygen Sensor [1-8]
AB: Fuel–Air Equivalence Ratio
CD: Current
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 link (which is 2/65536 * (256A + B) for the ratio)? What are the voltage and current values, and are they of any use to me in this situation?
Any help is much appreciated.
Depending on what you are trying to achieve, the oxygen sensor may not be the right choice, regardless of reading.
Oxygen sensors are usually located in the exhaust manifolds or exhaust piping. They read data from the exhaust gasses leaving the engine. This is helpful in determining if the engine is running correctly.
However, if you are trying to determine the AFR entering the engine, which is what it sounds like, the oxygen sensors would not be very helpful. A very high or very low reading could indicate that the intake AFR is not correct, but getting an actual number would be difficult.