A fustrated engineer with a question

2004 2.7L 4WD Silver Toy

I don't have a problem now, my son got me past an issue I don't understand.
I was getting a CEL code of P0136,faulty rear O2 sensor, after replacing the downstream O2 sensor. I must add the original sensor did not have nuts on the studs holding it to the flange. I cleaned up the stud threads, carefully with a thread die,, and the flange it was pitted from the exhaust leak.
My son found it was leaking exhaust at the flange. We/he cleaned up the flange with a file and after the reinstall, no code.
The original problem started with a P0420, poor catalytic converter efficiency, code. I get decent fuel mileage, 21 to 23 mpg, and haven't experienced any loss of power.
My question is: why would the computer label the O2 sensor faulty because exhaust was leaking at it's mounting flange?

 

Because CEL codes don't pinpoint what the exact, specific issue is. It tells you what system is recording a fault, it's up to you or the mechanic to figure out what exactly is the issue and where, then fix it accordingly.

You may find some interest in a YouTube channel called "SchrodingersBox." He has a lot of helpful explanations and it is super interesting, it's essentially fact based mechanics - focusing in on systems and doing extensive testing to fix issues as opposed to throwing parts at things and not understanding why the problem(s) persist.

Welcome to TW. Thanks for your service.

 

as said above, its pretty dumb sensor without much detail beyond things are not within the expected range it should be, regardless of why.

often you get a bad o2 code when its something else causing the sensor to be out of range such as a dirty MAF sensor telling the engine to run the wrong air fuel mixtures

 

A simplified explanation, and guess of what happened:
O2 sensors develop their own voltage from heat; the computer reads high or low voltage coming from the sensor to adjust air/fuel mixture accordingly.

With some of exhaust bypassing the sensor, it reads "lean," (or rather, "too clean ...") possibly enough to drop its voltage so low the computer tries to enriched the fuel mixture until another program in the computer says "Hey, wait a sec, this would make it way too damn rich. We can't just pour raw gas into the exhaust ... I bet the sensor's dead."

 

P0420 typically the cat isn't getting hot enough I'm told

because these bullshit little millivolt sensors throw codes over any anomaly & unless code is specific to a specific condition 1 P**** code thrown can be many things

 

I was going to answer with a bunch of info on P0420 codes, but i decided to put it into a new thread. Text below.

https://www.tacomaworld.com/threads/to-answer-some-common-misnomers-of-the-p0420-code.396763/

To answer your question more specifically, there are two possibilities. The excess oxygen from the leak could have thrown the sensor outside of the acceptable range that the ECU is reading. As the ECU reads the voltage generated by the sensor (simple o2 sensors act as a battery), a lack of voltage could of been read as a circuitry issue.

Also if the thing was hanging out of the port, maybe the wiring could have been damaged, or shorted. This could also happen internally.

 

Ditto. I see it so much on this forum that people rely way too much on OBD codes to tell you exactly whats wrong. Its just an indicator of a signal to the ECU being outside of a set range of volts, pulses, or counts.

If you have a scanner with mode$06, you can see the raw hexadecimal data and the upper and lower ranges that would trigger a attached code if that said number falls out of range.

Its a diagnosing tool, like a vacuum gauge or voltmeter, not a pocket mechanic. Know what they mean and how to treat them and it will make things much easier.

 

one last note is it also measures if the temp at the forward and rear o2 sensors is not the same which is why an exhaust leak can trigger the code in case it thinks you have a clogged cat