Downstream cat delete?

I’m beginning to think my downstream cats are bad, well at least one of them. If I were to cut the downstream cats, would it throw a CE light? Seems like it wouldn’t due to the o2 sensors only monitoring the upstream cats. If it does throw a code, how and why?

 

Is there more informative context you can provide? What DTCs are you throwing? What drivability issues are you experiencing? What is the relevant maintenance history?

In the meantime, my first bit of very gentle and candid advice is this.

May I beg your forgiveness, I offer this in a genuine tone of voice, but that is not a technically correct diagnosis. And with genuine respect, that is not a logically correct reason to remove a cat converter(s). Guessing is no bueno.

More info, please.

 

Yes, it will throw a code. They are part of the emissions and will throw a code if the ECU doesn’t get back the correct resistance in the circuit. You won’t find simulators being sold anymore but they can be made fairly easily. I don’t believe you would run in an open loop though but could be wrong. Smell will be a bit strong though.

 

If you carefully cut the downstream cats, no CEL will result, because the downstream cats are unmonitored. If you manhandle them, it's possible to introduce a leak at a gasket somewhere upstream and cause a lean code.

The cats also has some resonator function. Deleting them may give you some rasp at ~3000 rpm.

Now would also be a good opportunity to reroute the crossover pipe for more clearance.

 

+1, This is your answer. 02 sensors read and monitor primary cats not secondary.

 

https://www.tacomaworld.com/threads/i-threw-parts-at-my-truck-runs-the-same.863467/#post-30768481

Here is a thread I posted. It was almost a unanimous catalytic response.

 

For context this is how I’ve gotten to my conclusion.

https://app.truvideo.com/v/ufaxkmEQ?s=S+qUq/vFPQvo4kfIF3hiU1dZOKV5hkfS
Video from the dealer when I got a diagnostic.

And…

https://www.tacomaworld.com/threads/i-threw-parts-at-my-truck-runs-the-same.863467/#post-30768481
my other thread that had almost a unanimous catalytic converter/back-pressure conclusion.

 

Forgot to add your quote to my response. I’m at 50/50 right now on it’s okay to cut and it’s not okay to cut. I agree with you because they aren’t monitored.

 

It's more like 90% OK to cut. The 10% covers workmanship booboos that result in exhaust leaks upstream

 

If you’re set on gutting the cats, I’d start by pulling the factory Y-pipe. Using a long pry bar, you can chip out the factory cat shown on the left side of the image since it’s a straight shot. The other side is trickier, but you could cut a square section out of the cat body, remove the material from inside, then weld the piece you cut back in. I’ve seen others do it successfully on other makes and models. If you don’t have welding experience, I’d definitely take it to a shop as 3JOH22A mentioned because you don’t want to risk ending up with a leak. I would triple check that it's the secondary cats before going through the trouble. I don't know for sure but I would imagine the cats in the headers would clog first as those are the primary cats.

 

You will 100% get a CEL without a simulator. They are monitored by the ECU. They are NOT used for A/F regulation.

Here are the codes tied to the secondary sensors.

P0136
P0137
P0138
P0139
P0141
P0156
P0157
P0158
P0159
P0161
P0420
P0430

I’m not saying why you should or shouldn’t. I had to put mine back for visual inspections. Mine is a 14 but SAI has nothing to do with it. 06 and 14 ECU monitoring is still the same.

EDIT: I am incorrect and was thinking of the sensors in the y-pipe not the cats.

 

^False information or you're getting things mixed up. Front cats are monitored and can generate codes. Rear cats (the ones by the transmission, shown in tacokarl's picture) are not monitored. There are no sensors downstream of the rear cats, so it's physically impossible for the ECM to monitor them.

 

so to clarify my statement, I am thinking of the downstream sensors in the picture associated with a y-pipe delete. Thus, removal of a the sensor positions in the photo. Those sensors are not used in A/F but upstream cat monitoring. You are correct. Gut away or cut out but beware of backflow O2 contamination with a straight pipe or hollowed out cat. I think the worst thing would be a strong smell or drone worst case.

 

^Yes when doing a rear cat delete, usually you cut downstream of the O2 sensor, leaving the sensor in place. If @Brenenkw10 's state doesn't have visual emission inspection, I don't advise trying to hollow out the cat, as the manipulations risk more leaks. Better to just weld in a pipe.

 

OP:

Thanks for the updates and clarifcation! The YouTube vid from your shop was very helpful and I am convinced it proves your diagnosis as correct.

I would be a dollar and a donut the substrate, honeycomb material in your downstream cat has crumbled apart and is now causing both your rattle and your sluggish power.

At first blush, I thought "no way", surely the upstream cats would plug first. But then reality thwacked me upside my pea brain with this realization. The downstream cat, i.e., the cat downstream of the Y-pipe junction experiences TWICE the exhaust pass through (says to self "you dimwit"). So, duh, of course, by definition it has TWICE the likelihood of failing.


My updated suggestions & conclusions:

1) Others here are correct. The O2 sensors work off your upstream cats only. The downstream cat is not monitored in any way. Which kinda makes you wonder why it is there in the first place. Gut instinct is it was a late engineering change by Toyota due to then-current rapidly changing CAFE (Corp Avg Fuel Econ) and emission standards.

2) That means you can remove it and it should not affect your PCM controls as it relates to fuel trims, A/F rations, etc.... You should not even get any kind of "cat inefficiency" DTC code because that cat is not sensored.

Much much much more specifically, you can hypothetically consider "punching through" that downstream cat. Like with a piece of rebar with an angled tip. ANd shake out the internal debris and repeat. Punch it and wallow it out until there is nothing left inside. Whatever it takes. Hypothetically of course.

The hypothetical advantages of this approach is it is a NO COST solution. Except for your labor. Well, I would personally allocate a 12 pack of beer for such a job. But I digress. Also, upon re-installation, all of your OEM exhaust is still there from a visual inspection perspective. Some states are getting very intrusive with (bi) annual state inspections. A nit picky auditor could fail you for a state inspection because you are missing a cat - whether or not you are meeting tailpipe standards. If your state measures them like with a tailpipe probe or gas analyzer.

3) I would not recommend or suggest punching through your upstream cats. That can and probably will affect your tailpipe emissions and could also cause DTC codes for things like "cat system inefficiency".

HTH! Best wishes and good luck!

AE