Couple of random questions about oxygen sensors- relating to the upstream side.

If the sensor fails completely, will it always throw a code? Or only depending on why it failed?

What if the sensor is failing because of "contamination" or "chemical loss"?

I understand the computer will keep adjusting air/fuel mix based on the readings- is there a maximum or minimum variance (voltage or percentage or..??) if the sensor is failing before the computer uses a default reading? Or is there ever a "default number" used in any situation?

If the computer is adjusting air/fuel to compensate for a failing sensor, how does this affect the initial idle relearn after a battery disconnect? Let's say if for age alone the sensor can no longer detect accurate levels of oxygen (or none at all), where and how does the computer adjust to compensate?

And what would a failing or "dead" sensor affect besides gas mileage and emissions if the computer thinks everything is working- or at least not throwing any codes indicating otherwise?

I had trouble determining how many O2 sensors there are on my 4.0 OHV, it should be 3- I'm assuming that is including the one after the catalytic converter? Just making sure there isn't one I'm not seeing when I'm under the truck..

thank you for any input to my inquiries!

 

O2 sensors use a chemical reaction to measure Oxygen in the exhaust, this chemical reaction can ONLY take place above 650degF, this is why O2 sensors are Heated, 4 wires, now-a-days
This chemical reaction generates a small voltage, which is "the sensor voltage" that the computer uses
0.1 volt is high oxygen, often called lean
0.9 volt is low oxygen, often call rich
People often misinterpret lean and rich because they think of fuel, O2s can NOT see fuel, just Oxygen

A bad spark plug will cause a misfire in that cylinder, that means NO OXYGEN was burned up because there was no ignition, and that Oxygen is then dumped into the exhaust, upstream O2 on that side "sees" higher Oxygen level, so may set Lean Code, nothing to do with fuel, just high Oxygen because of the misfire

O2 sensors start to run out of chemicals after 100k miles or 12 years, which ever comes first
This causes FALSE lean, so computer tends to add more fuel to correct the lower O2 voltage, and that costs YOU money in lower MPG over the next few years, so new O2 sensors are basically FREE if you change them "on time", because you will have to change them when they set a code at 130k miles or 14 years, and that costs you 3 time more in extra fuel burned at that point

Yes, they will eventually fail outright and set a code

1993 and up V6 engines have 2 upstream O2 sensors, and then 1 or 2 downstream O2 sensors for Catalytic Converters, 1 or 2 depends on single or dual exhaust, and in later years O2s were used IN Cat Converters, so can have 2 on single exhaust system
So 3 is the minimum on a V6(or V8) but can have 4

O2s have 4 wires
one 12volt wire
one ground wire
For the Heater

2 sensor wires for Oxygen levels

O2s are NOT used on cold start, they need to be heated up by exhaust and heaters
O2s are NOT used at WOT(wide open throttle), computer ignores O2 feedback at WOT to give driver FULL POWER, for passing or ???

O2s are not "learned" sensors, computer uses their 0.1v to 0.9v as "the word of God", it has no other sensors to compare them to
O2s are the ONLY sensors that wear out, they have a specific mile or time limit, much like Batteries, tires and brakes

Yes, blown head gasket sending coolant into the exhaust, or leaking valve guides causing burned oil in the exhaust WILL shorten the life of O2 sensors

 

This is very helpful information.

Are the o2 sensors another brand specific part or are aftermarkets okay these days?

Now I'm perplexed however.. when i got this ranger in 2014 with less than 40k miles, the o2s should have already "expired" based on being 17 years old. I'm fairly certain they were still original at that time.
Somewhere around 130k miles, there was finally a cel for the o2 sensors and pulled these codes:
P1131, P0136, P1151
At the time, i used auto zone to pull the codes, and they were a bit less informative - now i reference this lovely thread for better descriptions - https://www.therangerstation.com/tec...trouble_codes/

the paper auto zone gave me read like this-
P1131 lack of HO2S11 switches - sensor indicates lean
P0136 O2 circuit (bank 1, sensor 2)
P1151 lack of HO2S21 switches - sensor indicates lean

This was in 2020 before i learned how many sensors are in my truck and where they are. I had a friend helping, and for whatever reason we decided to put a new sensor (bosch) in B1S1. I recall doing a battery disconnect to reset the pcm.. but the light never came back on for any o2 related codes.
I should have changed all 3, and i will be changing the other 2 very soon.

The scanner i now have still seems to not let me view live o2 data, or fuel trims - they always display the default values of o.o volts and -100% on all long and short term fuel info (which i also need to learn about!)

So until recently, i assumed they were all fine unless they threw another code. I also read some interesting articles about these sensors, and one bit stuck in my head about sending bad signals and thus making the fuel pump work harder than it needs to, which could wear it out faster.

*edit*
i am still not sure exactly what size my gas tank is - either 16 or 18 gallon because of "a gallon of reserve" or something..?? I've never run it out of gas completely but have filled just a tad over 16gal at most.
The gas mileage has always been around 12-14mpg since i got it, and this is my first ford and first truck so not much to compare it to. But now that I'm learning all this good stuff, wondering if i would have gotten better mileage if i had changed all the o2 sensors back in 2014..hmmm..

 

For OxSensors, strongly recommend FordMotorCraft or Bosch (Ford original supplier);
avoid cheap asian knock off copies; they tend to last a few months then die young.
A Bosch 15719 with 24i00 0m610 Long Cable+Plug is good to replace any Ford 4WireNarrowBand Sensor.

 

Thank you for that. Yay for me that the one i replaced was with a bosch.

When i replace the other 2 sensors - bank 2 upper, and the lower also, should i reset the pcm or let it be?

In the future I will replace all 3 at the same time