The downstream O2 sensor is there to monitor the cat converter. Usually when the converter dies the waveform follows the up and down switching of the upstream sensors. You need to run the truck until the downstream sensor finishes drifting down to see where it stabilizes. There are other tests to confirm a bad cat such as exhaust back pressure and temps using an IR gun. I would not replace a cat unless I was sure it was dead, most aftermarket cats are of crappy quality compared to the oem version.

The best way to check the injectors is an injector balance test. You use a pulse box to fire each injector in turn after charging the fuel pressure while watching the fuel pressure gauge to determine if they are all firing and injecting the same amount fuel. This does not tell you if the spray pattern is good just the amount of fuel being delivered. Injector lowside control and coil function is checked with a Noid light. Injectors that are getting proper low side firing control but do not have balanced fuel delivery can sometimes be cleaned up with injector cleaner. The best on vehicle procedure is special concentrated injector cleaner injected into the schrader while the truck is running using a canister and pump. Some advocate gas tank additives to do the same thing over time, common products are Seafoam or Chevron. Injectors are one explanation for high or low fuel trims but only one of many possibilities. An injector that is stuck open would be causing all kinds of problems and would cause you to fail the fuel pressure test outlined in the sticky as well as the injector balance test with the pulse box.

I am still running the factory injectors in my truck so I cannot comment on the performance change after upgrading to the newer spider style but many others here can.


George

 

Here is a good discussion of cats and downstream O2 sensors. Note with a bad cat how the downstream and upstream O2 sensors mirror each other and that with a good cat the downstream is more steady but not a complete flat line. Downstream also don’t always hover around the theoretical 0.45v depending on engine conditions and mnfg. Some will sit at a higher voltage for instance. Watch the video a few posts down (#29632):

https://www.scannerdanner.com/forum/...on-normal.html


George

 

Thanks for the feedback, George. I think I've got a list of things to check out now.

I'm wondering if the cat is clogged? I'm not sure.

It's been lacking power especially when merging into highway traffic, so that lead me down this rabbit hole looking at fuel trims.

 

Back pressure test is the sure fire way to find out.


George

 

Got back from an hour drive and here's O2 data I pulled in the garage at idle. Downstream is leveled out, but it seems high.

Fuel trims look better at idle. None of them are greater than absolute 10%.I should have got another screenshot of the top of the graph. I'll get more complete data.

I'm still trying to trace down this weird lack of power issue. Yesterday it road like a dream. No vibration and accelerated quick with little throttle input. Then today it shook a lot and could barely get out of its own way at 80% throttle. I'll have to get video of my water bottle shaking in the cup holder for reference.

If something in the transmission were causing a vibration that would shake the chasis at highway speed, would that also cause poor acceleration?

 

The point of the downstream O2 sensor is to monitor your cat efficiency. Here is a discussion of using the sensors to analyze your cat operation. All fuel delivery and ignition problems should be addressed first with good total fuel trims in both banks. When your computer sets a P0420 code it is doing something similar to this by manipulating the air fuel ratio and measuring the response time:


https://m.youtube.com/watch?v=vyVnhCIMDnw

Btw, the propane goes in after the MAF, just raw unlit gas.

Other Methods to induce a full rich to full lean condition are snap throttles or doing a driving test going to full throttle then zero throttle while your system is graphing (this one makes the most sense to me after propane). Others here advocate forcing lean with a vacuum leak. I have also seen snap throttles to go rich and then a vacuum leak to force lean. I do not think that there is a standard time lag for the oxygen storage of the cat but no lag is clearly a wasted cat, 1-3 seconds is probably marginal and 3+ seconds is probably good.

George