Rear O2 sensor readings bouncing around

[gmcman]

Given the age of the cat, I'm starting to guess it's effectiveness. I have fairly fresh sensors and even after 30 min of driving, I'm getting readings from .3 to .75 and this is under a slight steady load. I would think once the cat is nice and hot, even under a light load, that it wouldn't follow the front sensor as much and remain more steady around .45 to maybe .55.

The rear sensor doesn't mirror the front sensor but tends to try to follow it at a slower pace but doesn't reach the extremes as the front sensor.

 

[BlazingTrails]

That is how a narrow band O2 sensor works, the signal bounces back and forth. It is normal.


https://en.wikipedia.org/wiki/Oxygen_sensor

 

[gmcman]

That is how a narrow band O2 sensor works, the signal bounces back and forth. It is normal.


https://en.wikipedia.org/wiki/Oxygen_sensor

Front sensor I can see but I thought the rear sensor was almost steady when the converter was hot.

 

[Sparky]

Yes front sensor bounces. Rear shouldn't bounce a whole lot, should be relatively steady. Mine only has a very slight waver, unless I floor it then (as expected) the readings go to one extreme.

If it hasn't thrown the SES light yet for cat problems I'd just keep trucking, but be ready to replace the cat.

 

[gmcman]

I did a backpressure test just to be sure, everything looks good. I will check the temps fore and aft later today.

Idle and 2500-3000RPM

https://www.youtube.com/watch?v=EOXyBY6upWc

 

[littleblazer]

Sounds like your cat is not... Um... converting anymore. Even with excessive back pressure I would guess it to read a normal pretty steady line.

 

[gmcman]

Sounds like your cat is not... Um... converting anymore. Even with excessive back pressure I would guess it to read a normal pretty steady line.

If it isn't working as well, obviously not enough yet to throw a code. I don't think the computer reads the fluctuations as an error but extreme values. Back pressure at only .25-.3 PSI is good.

 

[littleblazer]

If it isn't working as well, obviously not enough yet to throw a code. I don't think the computer reads the fluctuations as an error but extreme values. Back pressure at only .25-.3 PSI is good.

I didn't mean you have back pressure, I meant with high back pressure it probably wouldn't fluctuate much.

 

[dfc739]

FWIW, here is a screenshot of Torque Pro on my phone. I have it set up to monitor the pre-cat O2 sensor above and post-cat O2 sensor below. When it's cold the two graphs are very similar but when warmed up the bottom one is basically a flat line. My phone likes to display the volume when taking screenshots...

 

[gmcman]

I checked the temp of the cat after a short drive and I'm not seeing a big jump in temp fore to aft. Idling for about 15 min, it's about 380 deg in and about the same out. After driving for 20 min, I got about 500 ish in and about the same out...which is kinda high for the inlet but not terrible as I just pulled over.

The rear o2 sensor does move around but when it stabilizes it's around .720. I can get it in the mid 600's but not for a long time. I will get a graph for you from Torque tomorrow as well.

As far as the volume showing, if that's a Galaxy...mine will do that also unless I hit the volume and the center button at exactly the same time, otherwise I get the volume also.

 

[gmcman]

This was my drive home, the pic idling is self explanatory. The rest it seems the cat is working but not up to par. Should acceleration at hwy speeds result in a flat line from the cat? I assume once hot it should be almost flat under all conditions under a load.

 

[Sparky]

Heavy acceleration will make it move around.