Downstream 02 Sensor Readings

My upstream 02 Sensor reads what I think is normal at around 0.995 to 1.015.

But my downstream sensor reads all over the place from 0.135 to 0.850 fluctuations every couple of seconds from the low end to the high end and everything in between.

Is this normal? No codes shown on my Ultragauge. Truck seems to run smoothly. Recently replaced plugs, wires, fuel injectors, cleaned throttle body and MAF.

 

Under what conditions (idling, city driving, highway, etc)? How do your fuel trims look? Variation is normal; not sure what is normal for your specific truck. Age? Mileage? Original cats?

 

2003 3.4L with 290,000 miles. Original cats as far as I know. Definitely see the downstream fluctuation when at idle. When driving the fluctuation is much less frequent and the number seems to increase when I'm on the throttle and decrease when coasting. Highest number around 0.800 when on the accelerator and as low as 0.015 when completely off the gas. I just hooked up the Ultragauge a few days ago so I'm still learning how to work it. I'll see if I can figure out how to display the fuel trims.

 

Based on that, and the sensor spec, I'm guessing your cats are about done. That's definitely wayyy too much variation at idle. If you don't already have one, expect a P0420 code to show up soon.

 

Good explanation of B1S1 a/f wideband sensor vs. B1S2 narrow band O2 sensor
http://forum.ih8mud.com/threads/narrow-band-vs-wide-band-oxygen-sensor-summary.168494/

 

The downstream 02 will oscillate on cold start until it gets warmed up. After a few minutes it should steady out. If not, as @tirediron said, your cat's approaching the end of it's life. Not a big deal unless you have to pass emissions testing.

 

Should it fluctuate that much when driving after warmed up?

 

Nope. That indicates bad catalyst efficiency and is indicative of a failing cat.

 

Since I don't live in a state that requires smog certification, I guess I'll save that $$$ for something else.

 

When the down stream O2 sensor remains steady that tells the ECM the Cat is no longer working.

Since when driving the output of the engine changes all the time. The signal is always changing.

As long as you have no codes Do Not worry.

 

To be clear, yes you will see VARIATIONS as you increase and decrease speed but at constant RPM and vehicle speed with no gear shifting taking place it should be steady. Likewise at idle, after the 02 heaters have heated up sufficiently, rock steady. If you're cruising at 50 mph and maintaining whatever RPM and are seeing oscillations (not variations due to engine load) it's likely your cat is approaching it's end.

 

Not exactly - if your O2's unresponsive all the time it's likely the 02 sensor is dead not the cat. A steady reading (outside of changing engine load) shows a good working cat.

 

The downstream O2 Sensor's primary purpose is to monitor the catalyetic converter function, they are also used on some vehicles for minor fuel trim corrections.
As @ArizonaBrian said the downstream sensor will "switch" rich and lean until the converters reach operating temp, once they get up to temp the downstream sensor should remain fairly steady at around 600-800mv except during abrupt RPM or throttle changes if the converters are functioning properly.

 

I would need to drive quite far to find a road that was flat enough to drive a steady speed at a constant RPM . For more then a mile or two.

I can only go by the numbers I have in front of me.

 

Isn't that Lambda? I only use the direct voltage reading.

 

Idle