O2 sensor testing

[FritzTKatt]

Howdy ya'll! Long time no see.

04 3l 2x, daily driver, nothing crazy. Throws p0420 code.

Testing down stream o2 sensor in hopes that it is bad, not a cat problem (f* you california).

I put it in the vise and heated it up to turning red, hooked up to the DMM between the back (POS) and grey (NEG) on DCV. It peaked out around .3v. The best I can find is that it should peak around .85-.9v.

Backstory, couple weeks ago I washed out the engine bay, next day it started acting up. Quick trouble shoot over the phone and i had the wife push home (couple miles). Upon inspection, it had a couple hard misfires, but on random cylinders. Checked and cleaned everything, threw new plugs and wires at it (was in dire need anyway). No dice, bench tested the coil pack, bad. Had *8* cracks in various locations. New coil pack and she runs like new, after figuring out the plug wire diagram in the book is wrong! Also found a bad idler pulley, so the engine is much quieter now.

30 miles later, starts throwing p0420. No change in performance, getting way better mileage than previous to the whole coil pack fiasco. Engine sounds great, drives very well. Having a CEL is obviously a no-go. Cleared code and it keeps coming back, even wiped the PCM to ensure its not a residual code. Which leads me to trying to test the o2 sensor, because I'm not trying to throw parts at it, but really hoping it's not the cat.

 

[FritzTKatt]

So, I'll concede that I did get ahead of myself and didn't bother to test another, which I have since done. Another one (from bank 2), hit .875v no problem using the same test method. So I'll assume my test method and bank 2 is good. Going to throw down for a new sensor given the situation now.

 

[RonD]

O2 sensors use a chemical reaction to detect Oxygen level in exhaust, so just like a car battery the chemicals get used up and they no longer work well.

Not sure heating them up would give accurate measurement of full function

P0420-Cat converter not cleaning exhaust very well
Yes, that could be bad downstream O2 but for that you usually get P0139-slow switching O2 Bank 1, sensor 2, or similar

Computer switches engine from Lean to Rich constantly, so upstream O2 sensor voltage jumps up and down constantly.
This Lean/Rich switching gets Cat converter to work, won't work well otherwise, and also tells computer that the O2s are all working, constantly changing voltages.
Downstream O2 will do it less but will still do it.

Hope it is just a bad O2 sensor, but not a waste of money in any case, 100k-150k is average life expectancy of O2 sensor

 

[FritzTKatt]

My thought process is that a bad o2 sensor will cause it to give a faulty reading of the cat's efficiency. It's got about 130K on the clock, so I wouldn't question the sensor failing so much as I would a bad cat.

What I fear is that the cat got messed up from a misfire dumping raw fuel down the pipe. I don't really see how that's going to happen in less than a half hour of operation, but apparently it does.

 

[RonD]

You would noticed if unburned fuel was the issue, the popping noises coming from Cat after you shut engine off would be causing damage.
The fuel itself doesn't "hurt" cat converter, but it ignites in Cat converter and causes it to super heat, melting internal parts which causes that popping sound from metal expanding when it gets too hot.

Oil and anti-freeze can hurt Cat converter, they block passages and cause a sooty coating which prevents Cat from doing it's job.

I have had to replace a Cat at 90k and others at 250k so hard to say 130k would be OK or not.
I guess you will find out if new O2 sensor doesn't solve the P0420

 

[FritzTKatt]

No issues with oil or coolant in the exhaust.

Light came back on last night, I'm stuck on duty so I can't check and see if it's the same code. Wife said that it's still running great, just got that light on.

 

[cp2295]

I don't think an open flame on the o2 sensor is necesarilly good? well who am I kidding the engine is shooting flames all over it.. If you have a scan tool you can see if they are being "lazy" (hits a certain voltage and kinda hangs out there for a second or too).

 

[AndyB.]

Testing an O2 sensor with a propane torch is a valid test. I have a 20 year old Actron sensor tester plus, and using a propane torch is a test described in the manual. You use the torch to get the sensor up to temperature, then apply flame directly to the sensor (removing oxygen) and take the flame away, and led's come on the tester to show the switching. You can also use the torch to clean carbon deposits from the sensor.

 

[cp2295]

Good info Andy, good to know for future reference.

 

[FritzTKatt]

Swapped the left and right upstream sensors. Same code.

Pulled the system parameters, I think there may be an underlying problem in the fuel system, because the downstream sensor readings are similar, indicating a strong cat on the right side.

Code tripped about 30 miles after clearing.

2,058rpm, 48mph, 33* spk adv, 28.6% throttle. Cruise was set.

STFT B1 .8%
LTFT B1 3.9%
STFT B2 -3.9%
LTFT B2 7.0%

O2 B1 S1 .310V
STFT B1 S1 .8%

O2 B1 S2 .740V
STFT B1 S2 99.2%

O2 B2 S1 .155V
STFT B2 S1 -3.9%

O2 B2 S2 .645V
STFT B2 S2 99.2%

What stands out to me is the b1/2 s2 volts being so similar, while the sensor 1 volts are so different. What I understand is that the o2 sensors produce more volts in the absence of oxygen, so the b1 is running rich.

Is the inefficiency code thrown because of a threshold volt or % change?

What do ya'll think of this now?

 

[adsm08]

My thought process is that a bad o2 sensor will cause it to give a faulty reading of the cat's efficiency.

Your assumption is flawed. P0420 is determined by comparing the upstream and downstream signals. The downstream signal should be fairly flat-lined in he .3-.6V range. When the signal from the downstream sensor starts to switch and make a wave form similar to what you would expect to see from the upstream it sets the code. A failed downstream sensor would set a different code.

 

[FritzTKatt]

What do you think about my assumption that bank 1 is running rich? It is literally double the V of bank 2.

The short term fuel trims also seem to concur with this, as the bank 2 is running almost -4% while bank 1 is almost +1%.

Think maybe the injectors aren't working as efficiently on bank 1 causing the need for excess fuel to get dumped in, to compensate? However that doesn't seem to concur with the LTFT where bank 1 is running 3% less than bank 2.

Maybe the driving style also has something to do with it? The wife doesn't have much of a commute and never gets up to highway speeds for any real length of time, so the cats probably never really hit full operating temperature.

I should probably run a can of seafoam on a 1/4 tank with a new fuel filter.