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what is the difference O2 sensor vs. A/F sensor?

8.6K views 8 replies 4 participants last post by  pjksr02  
#1 ·
2002 Sienna CE with 200,000 miles. a months ago, the check engine light went on, with code P0420.

At the same time, I heard the typical exhaust leaking sound. So I went under the vehicle, and found the downstream O2 sensor is loose - the mounting bolt is completely rusted away. So I temporarily wrapped the sensor back tightly with steel wires, and then cleared the code. But a week later it came back.

A search points to upstream O2 sensor on bank 1. But when I tried to purchase an O2 sensor, the listing says "A/F and O2 sensor", so I'm confused.

I got the same code 8 years back, I replaced the two sensors mounted on the exhaust manifold, one in the front engine bay, which was easy to replace, another under the vehicle and have to be removed with a special socket. The OEM sensors cost $200 each and of difference part numbers, not sure if they are any different from each other. I also learned then that Bank 1 is the one from under the vehicle, the one at the front is Bank2. Today non-OEM parts are far cheaper.

My question is: what is the difference between O2 sensor and A/F sensor. Are the ones mentioned above A/F sensor or O2 sensor? the Clifton repair manual says they look the same without saying what each is for. That confused me even more.

Can some explain a bit what the difference are?

thanks much!
 
#2 ·
I could get into intricate detail as an engineer, but suffice to say they are technically different although the operating principle is similar.

The upstream "Wide Band Air/Fuel Sensor" is technically an oxygen sensor, but an expensive one that is more sensitive and outputs a higher range of voltage for better discrimination by the ECU. It's purpose is to fine tune the level of ingested air and fuel thru regulating the dwell time of the fuel injectors based on the raw output of combustion

The post catalytic converter unit is a cheaper Oxygen Sensor designed to monitor final exhaust and be a check on the efficiency of how your converter is doing it's job. Although it's output is used to a small extent by the ECU continuously, it really comes into play for about 1 minute once a drive cycle when the ECU runs a Cat Efficiency Test. At that time the ECU takes the fuel mixture intentionally 30% rich, and then 30% lean, as a 'stress test' for the converter. If the rear oxygen sensor says the cat passes, all is good. A fail and then a pass the next day is generally OK. Two consecutive fails and you get a Check Engine Light, and a P0420/P0430 code logged.

Sufficient detail?
 
#3 ·
thanks so much for the details!

So the ones I replaced 8 years back must have been A/F sensors, not O2 sensors? I read that the ECU of my MY has a sort of bug that cause unwarranted P0420 code. I wonder if it can be upgraded per software update.

Given the fact that the P0420 code happened when the downstream O2 sensor was loose and exhaust leaking, is it likely that sensor needs replacement? or should I check the upstream O2 sensor first?

BTW, how many O2 and A/F sensors are there all together on a 2002 Sienna CE?

thanks much!
 
#4 ·
If there is air getting into the system from a leak, then the post cat sensors will never read the actual oxygen content of the exhaust. Fix that situation first. New pipe, a repair what's rotted out.

I don't know off-hand the placement of converters on the older Gen-I vans, and that's often the key to sensor placement.
 
#5 ·
When my O2 sensor stud rusted off on my 2002, I got a new sensor with gasket, and bolted it on with a 4" pipe clamp from Home Depot.

Your wire method can work, too. I'd suggest a new sensor, if the P0420 doesn't clear soon.

Find your correct part number at Densoaftermarket.com, and for best results, use a Denso sensor.
 
#6 ·
Be assured your problem is common. Just don't go buy a new catalytic converter, to try to solve your problem.

 
#8 ·
thanks so much for share these knowledge. wanting to be surer, I bought a Bosch 1150 scanner that can display O2 sensor graphs. There are three sensors O2S11, O2S21 and O2S12. I believe the O2S11, and O2S21 are A/F sensors, which are in the range of 3.15-3.25 volts , O2S12 is the downstream O2 sensor. Since O2S12 is varying only between 0.045 and 0.088 volts, It must be the defective one. Right? But I don't understand values listed in the lower-right box. What do they say? thanks much!
46748
 
#9 ·
When my sensors were bad, I wasn't able to make much sense out of the outputs. I do remember they jumped around a lot, but were much smoother after replacing the sensors.

There are some resistance parameters and general graphs in the repair manual. That's still around online for download.