My fathers O2 (maybe) sensor threw a code so I bought a direct fit Denso to find that the plug was not keyed correctly. Tried a CA emissions one. Not correct. So I checked Denso's catalog and they list an oxygen sensor and an "air/fuel ratio sensor" of which I've never heard the terminology. The listing for the A/F ratio sensor is keyed correctly but what throws me is that for the same car, a 2003 Camry 2.4L, they list BOTH an O2 and A/F ratio sensor for upstream. Can anyone chime in on this? Does this car have in effect, three O2 sensors or is it that some models have one or the other?