Help, can't get P0420 code to go away

Good morning,
I need help, and my patience is running thin with my first gen. My truck is a 2003 2.7 4x4, and for the last couple of years I've had nothing but issues with this code. I've had multiple people tell me multiple things and multiple diagnosis (I don't have a real good code reader, so I've taken it to a local mechanic that specializes in toyotas and has a good code reader) and following is the list of things I've replaces and/or tried to remedy:
- LC engineering header (I've adapted a 2.5 inch exhaust pipe to it for a 2.5 inch exhaust)
- Mangnaflow converters (year and a half old)
- OEM Denso 02 sensors and gaskets (about 2 years old for sensors, gaskets were just replaced this weekend)
-new oem exhaust manifold gasket
-new oem egr tube gasket
- new stainless steel 2.5 inch exhaust pipes ( done this weekend)
-stainless steel high flow muffler (about a year old)
-v band connectors (installed this weekend, was tired with factory connectors and I've used these before and never had a issue)

So pretty much this is my laundry list of items I've replaced and my code just came back on this morning, my code reader says P0420 and the only "light" my reader is flashing along with the code it a "EVAP" light.
Ontop of that, since installing my 2.5 inch exhaust this weekend, when I come up to traffic lights my idle drops below 500 rpm and the truck shakes pretty good.
If anyone could offer some help on where I should look to next, please guide me in the right direction on where to look next, I'd greatly appreciate it because I don't know where to look next with this thing. I want this CEL off and I am tired of dumping parts and money into my truck that doesn't fix it. Thank you

Matt

 

Most common is exhaust leaks. What I’d seen in the past also is bad injector(s) or a dirty MAF which can contribute and trigger the 420

 

I had the same code P0420. I took a shop vac reversed the hose to blow air out and stuck it in the tailpipe.

I then used soapy water to find a Crack around pipe to flange on 1st cat.
I Welded it and have had no problems since.

 

A few things It could be.

1. The aftermarket cats could be the issue especially high flow cats. They work but not well enough to the ECUs OEM factory standards of regulating the fumes. For example my 10th gen civic if you replace the downpipe with an aftermarket catted or non catted they trigger check engine lights for P0420.

2. If you have any exhaust leaks it might set it off

3. If your ECU is faulty it might be that. Believe it or not my 1996 2.7 had a faulty ECU that kept saying my monitors werent readying up for 02 sensors etc. I replaced the ECU and it fixed the issue and the problem never came back.

Regarding the car dropping below idle honestly sounds a little more complex and I don't really have an answer for that and it may or may not tie into the p0420 code.

 

For vehicles that are driven off road an Underdog Racing Development rear O2 sensor simulator can be installed inline to eliminate catalyst related malfunction codes.

 

I think that they no longer manufacture and sell those defeat devices.

If i remember correctly the rear O2 sensor screws in, if possible and if there is room auto parts stores sell a treaded spacer. What I am hoping for is that it you get the rear O2 sensor out of the direct stream of exhaust gas that it might be just enough to fool the PCM into thinking everything is OK. It is a relativley complex series of checks that the PCM uses to determine if the CAT is working or not.

 

That would be a defouler and those are usually hit and miss. Im leaning towards the aftermarket cats being the issue. Probably might have to go back to OEM if thats the case.

 

Agreed, $20 is cheaper than $200+ for testing purposes

 

P0420 is simply the ECU looking at pre- and post- cat values. Post-cat is showing not enough damping from pre. That should narrow it down.

 

Same

 

Thank you for all your suggestions, I will check for leaks, however I just put my code reader on the truck and got some numbers off some of my different items at idle after warming up:
STFT B1: bouncing between 7.0-9.4
LTFT B1: -0.8%
Eng rpm: 700
MAF: .35
02 B1 S1: 3.288
02 B1 S2: .505-.830 (number bounces)

Everything from what I've researched is in normal operating range.... now I am really lost

 

its the cat thats making it throw the code paired with possibly fouled up 02 sensors im almost sure of it. Remove the 02 sensors and inspect them.

just because they are two years old doesnt mean they cant be dirty. I would remove both inspect and clean them, theirs videos online how to do so.

also I would remove the mass airflow sensor and clean that up to help with the stumble at idle as well as removing the spark plugs and checking to see how they look.

Try all the cheap and free options first before you throw anymore money at this thing.

Worst case you have stuck injectors dumping too much or little fuel causing it to run rich or lean, faulty fuel pump, or the fuel filter is clogged.

 

As Treyus30 pointed out the rear O2 sensor should be a dampened signal, it looks like your front O2 sensor is an air fuel sensor and not a plain ol O2 sensor, much more sensative, think saw tooth pattern vs a long smooth ocean swell.

 

I would also try to see what conditions trigger this.

Pull the battery ground for a minute, put back on, verify code gone, then let the truck warm up to operating temp in the driveway. Then go on a very low throttle drive around the block a few times. See if it comes on. If not, then increase from there until it does. Might tell us something because I agree, your idle variables seem normal.

 

your cat monitor sensor is probably switching rapidly compared to your 02 sensor voltage, indicates your cat isnt doing its job any longer....

 

In my experience, the rear O2 will pop code if cat working below 90% efficiency
Meh.
I have used the anti fouler method on 4 vehicles GM(2)/subaru/honda
Just as long as u have clearance….works everytime

the other thing I was thinking
I put headers on a Subaru
Kept popping the code
I spoke to a company JetHot(ceramic coating vendor). Tech there said that chance that exhaust flow so free flowing, too cool. As apposed to 90degree bends in stock exhaust that heats up gases read by O2 rear. Made sense
JetHot prices based on 4/6/8 cylinders
These Subaru boxer engine headers were huge…..they only charged me the 4cly price
Fixed my p0420 bc exhaust gases were hotter bc of coating
Thinking wrapping headers would have same effect

 

Tried the anti fouler on my brothers '12 Fiesta and it didnt work, he was so pissed, $1k for a cat. I told him stop playing with that thing and put a cat on it, he kept playing and it set a permanent code, that was VERY time consuming to get rid of, thats what he told me as I wasnt going to "fix" it after I advised him.

 

Yeah
I guess my caveat i should of mentioned, all 4 of those vehicles were 2005 and older

 

Do the leak test using a shop vac and soapy water. Will take you like ten minutes to do and either rule out or resolve this problem.