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Diagnostic Help - P0420

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    Hello everyone! I've been a lurker for quite some time, but need some help diagnosing an issue. I will do my absolute best to provide all the relevant information possible. Any suggestions, requests, or ideas would be greatly appreciated. There's some code(s) I'm having trouble getting to the bottom of, and I'm not sure where to start without biting an expensive bullet. I've done most of my own maintenance thus far, and while I think I do/know more than the average joe, extremely technical concepts may need a bit of explanation. Thank you so much in advance for anything you can bring to the table!

    Car Info:
    -2006 Scion tC (manufactured 02/06)
    -Roughly 175k Miles
    -Regular Oil Changes/Maintenance

    Previous Service:
    -100k(ish) Miles: My dad replaced one of the O2 sensors...not too sure which one; he thinks it was the 'upper.'Don't know if this was due to a DTC code.
    -115k Miles: P0420 shows up. Told by mechanic not to worry about it. I don't worry about it. Code generally comes on every few hundred miles after it's cleared.
    -150k Miles: Couldn't find anyone to inspect my vehicle due to P0420 code that hadn't been an issue (getting inspected) in the past. I foolishly took the CAT off, thinking it was like the one on my Grandpa's old truck that he beat the hell out of and flushed with mineral spirits to 'clean.' I soaked in mineral spirits but obviously couldn't beat anything out of it. Reinstall manifold/cat 2-in-1 (thanks scion for making this so expensive) and forget to replace lower gasket leading to pipe.
    -160k Miles: P0420, P0441, P0455, and P0456 appear. Cleared.
    -170k Miles: P0420 remains, no other codes show. Exhaust manifold gasket between manifold heads and engine is replaced.

    Known Issues:
    -Obviously the cat code that's been keeping the 'check engine' light burning for years.
    -The gas cap doesn't lock all the way. It will click forward, but can be untwisted with little to no effort. (I know..I know...should have replaced a long time ago)
    -The lower gasket leading from bottom of cat/manifold to the pipes.
    -The air box/cleaner is kind've jumpy. It has a bit of movement to it; I think one of the lower brackets broke/rusted out.
    -The hose leading from the intake to the air box/cleaner has a bit of play at the seal. The meshy/rubber seal looks to be mostly sealed, but there's some play in the hose moving in/out of the box itself. Not sure what the condition of these need to look like.
    -Last time I took the MAF off, I think my dad may have cleaned it with something other than MAF sensor cleaner. Additionally, 1 of the screws holding the MAF connection in place stripped on the way in...I didn't back it out. Seems to be tight though.

    Symptoms:
    Jumpy acceleration after 3K RPM. See video attached. Any time theres half-3/4 throttle involved, from almost any speed, it jumps and lurches as I climb up the tac. Started ~5k miles ago
    Some loss of power/fuel economy across the board.
    Swishy sound when I do heavy accelerating; does not matter the speed. See attached video. No knocking/clicking. Literally sounds like a helicopter if all you could hear was air. For some reason I thought it was lose splash guards/plastics...mistake number 39382 while learning how to diagnose my first vehicle. You don't have to forgive me because I've still not forgiven myself.

    P0420 Freeze Data:
    Fuel Sys 1: CL
    Calc LOAD (%): 40.7
    ECT (F°): 188
    STFT B1 (%): 0
    LTFT B1 (%): 3.1
    Eng RPM: 2740
    Veh Speed (mph): 71
    Spark Adv (°): 32
    CAT (F°): 60
    MAF (lb/min): 3.556 (5.926666 g/sec if my math is right)
    TPS (%): 27.0

    I'm not sure really where to start. I've heard everything as simple as the gas cap, to the O2 sensor(s) needing to be replaced, the whole cat needs to be replaced, the intake is bad, the maf is bad, the gasket on the pipe is causing all this, and was even made an offer of $300 to sell the car since it was "likely totaled and ruined."

    Any ideas of where to start before I shell out mine and my ancestor's life savings on replacement parts till I can figure out how to make the code go away? Like I said, I've (fortunately and unfortunately) done most of the maintenance on this car myself, and have had to learn as I went along. Any advice/ideas you may have would be fantastic. If you need more diagnostic information from me, please don't hesitate to tell me what you need.

    Thanks everyone.

    <iframe src="https://player.vimeo.com/video/336507615" width="640" height="1138" frameborder="0" allow="autoplay; fullscreen" allowfullscreen></iframe>

    <iframe src="https://player.vimeo.com/video/336507637" width="640" height="1138" frameborder="0" allow="autoplay; fullscreen" allowfullscreen></iframe>






    I’m aware of all the potential things it could be. I’m looking for further diagnostic advice on trouble shooting one or more of the several parts listed.
    logann2292 wrote:
    I’m aware of all the potential things it could be. I’m looking for further diagnostic advice on trouble shooting one or more of the several parts listed.


    MRLUV is 100% correct. And if you read your own list of problems that exist on your car, you already have listed many on that list that cause a P0420 code! Not to mention you said your gas cap isn't tightening and P0420 is an emission evap leak. Get a new gas cap, STANT 10839 is a good one, works and is cheaper than Toyota's.

    you also have leaks in your air intake system, I can go on.. fix those problems first, then see if the P0420 goes away..

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