Need knowledgeable O2 Sensor Feedback

[tripn4days]

Long story short I am having a few CEL problems that started with towing a 4700lbs load a few weeks back. During tow truck ran fine for a mile and half and then just started misfiring and running really rich. Turned off and back on, problem went away for another mile and half then returned in same fashion. No idea the codes, but CEL was flashing at the time. Reset the CEL and been fine, but now code P0420 has popped up twice since then.

I'm using OBD scanner with Torque app which graphs O2's output, but I'm not sure what I'm looking for. I get that O2 flip flops between rich and lean and that my graphs should show high and low spikes, however my graphs only look pointy on the rich side of the graph, down near .2 or so. At the lean point, around .8 or so, the graph flattens out and then slowly "rolls" downward on an exponential-looking curve. Then it bottoms out and spikes back upward and repeats.

Is this normal?

After reading up on how O2's work, I was expecting to see more symmetrical spikes at top and bottom. Also, reading both front O2's simultaneously, I see quite a bit of variation between the two. Sometimes the ranges differ, other times the spikes and sizes of the flat spots differ and then othertimes both their graphs more or less match. It doesnt matter whether I'm driving or just idling, but the variations most often and most largely occur while driving.



So here's my theory; please correct me if I'm wrong... Going back to my strange towing issue, I'm wondering if this slow lean-to-rich switch is responsible the weird towing behavior I experienced? If the O2 is slow to react to lean conditions, and truck needed MORE fuel b/c it was towing a decent load, would that scenario prompt O2 into open loop operation, in turn dumping fuel, cooling catalytic and throwing the blinking CEL?

What do you think? I've never had a CEL before towing this heavy of a load. And now, despite being reset, I've had P0420 pop up twice...

Thanks Guys!

 

[The_Roadie]

You're on the right track. Wish you'd thought to share the mileage, because over 100K and O2 sensors tend to get crappy just like plugs. If you're over 100K, did you change the plugs? You should immediately schedule a sensor swap if they've never been changed, because a dirty sensor is a sluggish sensor, and you're describing sluggish data behavior.

 

[tripn4days]

It's a 2008 with 120k, which I imagine contains original O2 sensors. I picked up the truck last July with 116k and changed plugs, flushed all fluids, etc, but did not change the O2 sensors.

I guess either way, when it comes to emissions $%#@ the O2's are proabably the cheapest place to start. I tried running OBD app on my Volvo to compare the O2 graphs, but ELM327 doesnt like Volvos...

Thanks for the info Roadie!

 

[Wooluf1952]

I'm not sure about the 5.3L, but the 4.2L I6 does not work well with anything but the AC Delco O2 sensor.

 

[Chickenhawk]

In my view, your readings are not too far off an aged O2 sensor. It is unlikely that your O2 sensor has caused misfires serious enough to cause your light to flash. Those are serious misfires. I would speculate you are using the wrong spark plugs or you have lunched one or more coil packs.

Based on the rear O2 sensor sometimes matching the front one, I would also suggest you may need a new cat converter soon. But that's just a guess.