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#2617533 - 05/03/12 10:02 PM Oxygen Sensor vs Air/Fuel Ratio Sensor
FusilliJerry82 Offline

Registered: 06/03/06
Posts: 779
Loc: Richboro, PA
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?
2000 Protege 1.6 215k - Mopar Max Pro 10w-30/Mopar Oil Filter

#2617575 - 05/03/12 10:42 PM Re: Oxygen Sensor vs Air/Fuel Ratio Sensor [Re: FusilliJerry82]
Scott_Tucker Offline

Registered: 12/31/09
Posts: 894
Loc: Santa Cruz, Ca.
In a nutshell, the A/F ratio sensor is more accurate than an O2 sensor. They use the A/F sensor up stream because they use that to check the air/fuel ratio. The one after the cat is simply to verify the cat is working so an A/F sensor would be overkill.
2012 Honda Civic Si, Dyno Blue Pearl

#2617584 - 05/03/12 10:52 PM Re: Oxygen Sensor vs Air/Fuel Ratio Sensor [Re: FusilliJerry82]
yonyon Offline

Registered: 03/06/12
Posts: 3492
Loc: NJ, USA
Yeah, if they list both an upstream O2S and an AFR for Cali emissions, the upstream O2S is probably a catalog error.

#2617597 - 05/03/12 11:07 PM Re: Oxygen Sensor vs Air/Fuel Ratio Sensor [Re: FusilliJerry82]
yonyon Offline

Registered: 03/06/12
Posts: 3492
Loc: NJ, USA
The AFR is a wideband O2S. Instead of having an output signal that is a voltage above/below 450mv to indicate rich/lean, a current is applied to a sort of, well, half of an O2 sensor built into it to force the main O2 sensor within the AFR to signal 450mv. The direction of current flow indicates rich/lean and the amount of current required indicates how rich/lean the mix is.

They work wia the same basic mechanism, but with an extra layer on top for the wideband/AFR sensor allowing for much greater resolution. Because the regular O2S varies voltage as a signal and the AFR uses varied current as a signal there is no way to make one work in place of the other despite the plug looking the same. File off the bumps and swap terminals around as much as you want, it can't work.

Knowing how far off the mix is from stoichiometric rather than just rich or lean allows the ECU to correct the mixture much more quickly than just moving it a hair and waiting for a response from the O2S. This also offers the engineers an option of using a mix that is "a little rich" or "a little lean" in certain operating conditions without doing the "drop to open loop and hope for the best" trick.

#2618073 - 05/04/12 02:58 PM Re: Oxygen Sensor vs Air/Fuel Ratio Sensor [Re: FusilliJerry82]
mechtech2 Offline

Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
yonyon is right.
Upstream and downstream O2 sensors are often identical, just with different length leads.
Many O2 sensors are ON/OFF, but a wideband varies with theO2 readings.