Check the doughnut gasket between the header and the midpipe.

 

I have that same code. All I’ve done is to change O2 sensors but that didn’t fix the issue. Something I’ve noticed is that it comes on after highway drive.

 

Intake manifold gasket ever been changed?

 

Thanks, guys. Not sure about the intake manifold gasket because I just got the car 6 mos ago. Are you saying I may need to change it? I’ll check the doughnut gasket.

 

Can you tell if it's orange by looking at the edge? If it's not , it hasn't been changed and you should change it. It causes a few trouble codes from vacuum leaks...

 

Code po420 is catalyst inefficiency code. 9 times out of 10 it is the cat converter. You have to confirm it’s actually the cat convert by using a scan tool and graphing both the upstream and downstream oxygen sensor. The job of the oxygen sensor after the cat converter is to let you know if the cat is working or not.
What happens is you have pollutants going into your cat converter. They are detected by the oxygen sensor before the cat converter. When graphed in the presence of oxygen the before oxygen sensor should move up and down.
the pollutants are cleaned up by your cat converter and nothing should come out. So your after cat oxygen sensor will tell you if cat is bad or not. When you graph the after cat oxygen sensor if it see pollutants the the sensor when graphed will move up and down. Meaning cat is bad. If your cat is good no pollutants will come out of cat and when sensor is graphed you will see a flat line.
So that is why with po 420 which is usually a bad cat converter to confirm it you must use scan tool and graph upstream and downstream oxygen together.
Hope this helps.

 

Code po420 is catalyst inefficiency code. 9 times out of 10 it is the cat converter. You have to confirm it’s actually the cat convert by using a scan tool and graphing both the upstream and downstream oxygen sensor. The job of the oxygen sensor after the cat converter is to let you know if the cat is working or not.
What happens is you have pollutants going into your cat converter. They are detected by the oxygen sensor before the cat converter. When graphed in the presence of oxygen the before oxygen sensor should move up and down.
the pollutants are cleaned up by your cat converter and nothing should come out. So your after cat oxygen sensor will tell you if cat is bad or not. When you graph the after cat oxygen sensor if it see pollutants the the sensor when graphed will move up and down. Meaning cat is bad. If your cat is good no pollutants will come out of cat and when sensor is graphed you will see a flat line.
So that is why with po 420 which is usually a bad cat converter to confirm it you must use scan tool and graph upstream and downstream oxygen together.
Hope this helps.

Thanks! Yeah, that’s what I figured. Gonna take it to my mechanic at this point and let him run the scan

 

Does your Corolla burn oil (wet oily mess in the tailpipe is a bad sign)? That can plug up or coat the cat substrate. Assuming you have no intake or exhaust leaks (especially intake manifold gasket) and your O2 sensors are new DENSO brand sensors, you can remove the section of the exhaust with the cat converter (it's not hard to do). You should be able to view into the exhaust right before the cat and see if the "honeycomb mesh" substrate is plugged up. If you have a video boroscope (you can get them cheap now), you can feed that into the O2 sensor bung opening and take a view of the back side of the cat substrate also. If you have no intake/exhaust leaks, and properly functioning O2 sensors, and the cat isn't plugged up, then your cat is probably no longer efficient enough. There are cheap replacements you can get ($130 on Amazon) but I have no idea of the quality. After about 350K miles or so, my cat gave up the ghost. A rear O2 sensor replacement at around 250K got rid of the P0420 for a while. So, around 350K I used a spacer at the O2 sensor bung opening and the P0420 went bye bye. My cat substrate was very clean, btw (don't know if it was previously gunked up, because I ran lacquer thinner before removing it).

Make sure your PCV is in good working order and that you don't have a leaking valve cover gasket.

Lastly, some of the 2004 (and 2003 or 2005.....can't remember anymore) had a TSB issued stating that a PCM/ECU updated calibration was available to give more flexibility to the cat efficiency threshold to help solve the P0420. It costs one hour of labor from your local Toyota dealer to do that recalibration (the TSB says it takes 8/10 of an hour, but the dealer is gonna charge you the whole hour). At least that's what they did to me even with me pointing out it was 8/10 of an hour on the TSB. I think I paid $107 several years ago. Make sure they give you a sticker which shows the recalibration was performed. You put the sticker on the car somewhere (underside of hood or perhaps in the door jamb).

Here's the link to the $130 cat exhaust assembly on Amazon. It shows "currently in stock". 83% 5-star reviews, 13% 4-star reviews, 5% 3-star reviews. Zero 2-star and 1-star reviews.

Amazon.com: TED Direct-Fit Catalytic Converter Fits: 2003-2008 Toyota Corolla/Pontiac Vibe/Toyota Matrix 1.8L FWD 1ZZFE: Automotive

Buy TED Direct-Fit Catalytic Converter Fits: 2003-2008 Toyota Corolla/Pontiac Vibe/Toyota Matrix 1.8L FWD 1ZZFE: Catalytic Converters - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Here's an example of an O2 sensor spacer (only use this if your cat isn't plugged up):

Amazon.com: Stainless Steel O2 Oxygen Sensor Spacer Expender M18 x 1.5 Thread 45mm (1.78") Length SS 304: Automotive

Buy Stainless Steel O2 Oxygen Sensor Spacer Expender M18 x 1.5 Thread 45mm (1.78") Length SS 304: Automotive - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

i had this problem before and everything pointed to a new catalytic converter but after reading in here there is this fix to your rear oxygen sensor that solve it. Hope it helps.

hxxxs://www.ebay.com/itm/2X-O2-OXYGEN-SENSOR-EXTENDER-EXTENSION-SPACER-M18-x-1-5-02-BUNG-ADAPTER-OBD2/173384205025?epid=2291277876&hash=item285e8102e1:g:HJEAAOSwSNddmWjC

 

Just so readers understand, from post #9 the O2 sensor spacer that I used at around 350K miles does get rid of the P0420, but it is not a repair for it. You still have inefficient catalyst performance, but the spacer tricks the car's main computer into thinking that the cat is functioning efficiently. I recommend making sure all the other things are checked/fixed first before resorting to the O2 sensor spacer (or replace the cat if you've got the money).

 

I stand corrected, my car had 125k when this happened @350k must definitely need a new cat converter.

 

Just a data point. This spring I finally replaced the 140 k miles 04 CE's rusted out (by front) cat with Bosal (Canadian) brand found on Ebay for ~$90+. The muffler shop charged $55? for install, which a lift is handy for. Had some $ in the bolts/springs/etc (Summit Racing/Walker stuff), but gasket OK.. That quieted things down nicely. OEM cat welded to intermediate pipe and it's 50 mm diameter. Summit had it wrong and my $7 stainless band clamp for 2.25" wasted. Installer just used sleeve and mild steel clamps. did get ONE PO code. That's when I started looking around.

I think I'd go for a Canadian sourced cat over unknown or Asian provenance.

 

Um, my bad. OEM is 55 mm AFAIK. Kinda close to 2.125", not as big as 2.25".

 

Oh, yeah, haven't tried to sell old one to jy yet. Kinda hassle last time, but surprised at the price.

 

Hi, do you think that bad spark plugs can cause this code?

 

No, not probable (Edit: It is listed as a possible cause, though, in code explanation charts online). Bad spark plugs would give you a misfire code, like P0300 , P0301, P0302, etc. This code (P0420) is caused by an inefficient cat, faulty O2 sensors (rear sensor in my case) exhaust leak, not updated ECM calibration, etc.

 

Also caused by air intake leaks, fuel injector problems/leaks, or dirty air filter.

I'm looking at something online that says that yes, bad spark plugs can cause the P0420. I've never actually had anyone that I know of that had that issue, though. Are you also getting any misfire/spark plug codes?

How many miles do you have on your spark plugs? Which spark plugs do you have?

 

I replaced the downstream oxygen sensor and that did the trick!

 

Less than $20 USD online. Here's an example:

Amazon.com: Kizen LaserPro LP300 Infrared Thermometer Non-Contact Digital Laser Temperature Gun with LCD Display -58℉～1112℉(-50℃～600℃) Adjustable Emissivity (NOT for Humans): Industrial & Scientific

Amazon.com: Kizen LaserPro LP300 Infrared Thermometer Non-Contact Digital Laser Temperature Gun with LCD Display -58℉～1112℉(-50℃～600℃) Adjustable Emissivity (NOT for Humans): Industrial & Scientific

www.amazon.com

 

John. I that the laser thermometer you have the one from the link. Thanks

 

No, the one in the link is not the one I have. It's just an example. They are mostly the same.

 

it needs a new cat
i used a o2 spacer, code hasnt come back yet, did this few months ago

 

If the O2 sensors have been replaced with the correct Denso sensors, and there are no intake or exhaust leaks or fuel injection problems, then it could be the cat, but sometimes cats can be cleaned and then they work again. At 386K miles, my cat is squeaky clean and just isn't working any more. The cat can be dropped and the mesh media inspected to see if it's covered in soot/oil/carbon/gunk. If it's clean, then yeah, it's probably dead. I previously had intake leaks, causing the computer to dump more fuel possibly fouling my cat, and I personally think neglecting the valve cover gasket didn't help either. I also have a spacer on the rear O2 sensor. A cheap cat can be had for $125 on Amazon and it has good reviews. I give my cheap $8 spacer good reviews, though. Maybe I'll replace the cat some day. Maybe not.

 

You can disconnect the midpipe from the header and shine a light to see if the honeycomb is in tact. They dont last forever.

 

Right, and it's an easy job to do. I was able to reuse the donut gasket without any problem. Just shine a light in there and you'll see if the mesh media (honeycomb) is plugged, clean, or disintegrating. Mine was perfectly intact and clean, but DEAD.

 

John when you checked your mesh media to see if it was plugged did you use your inspection camera.

 

You can easily see the front side of the mesh media with a flashlight. I used a boroscope camera through the rear O2 sensor bung to view the back of the mesh media.

 

Hi there was wondering if i could get some advice on my toyota yaris 2004 it had been fine then let it to sit for a momth or two.it has the same code(sluggish performance,draining my fuel definitely running rich due to you can smell it)I remove the code and the rpm goes back to normal but as soon as I drive like an hour or so it goes back to it all. I've checked spark plugs there fine checked the air intake that's fine and the hoses which are fine there's no distinct catalytic smell at all just wanting to see what you guys think it could be.

 

I'm assuming you mean the code is P0420? If not, you should post a new thread to ask for help. If yes, do you have any other codes? Sluggish performance is one problem you'll get with a plugged-up cat. You should drop the cat (not hard to do) and shine a light in there and take a look at the mesh media. If you don't have a boroscope for the rear O2 sensor bung hole, you can shine a direct beam light in the bung hole and see if you can see it freely shining through the mesh media at the front open end of the pipe.

 

The only thing that makes it not so easy is if you have a rusty car. If it's not rusty, dropping the exhaust is no big deal. Removing the rear O2 sensor is easier BEFORE dropping the exhaust. Be warned, though, sometimes the rear sensor strips the bung hole threads upon removal.

EDIT: I just noticed that you said your car is a Yaris, so although the concept is the same, I don't know exactly what the exhaust looks like on that car. Is there not a lot of traffic on the Yaris forum and that's why you came onto the Corolla forum?

 

So i haven't checked the cat but have checked the sensor reading and there fine the 2nd one does spike at all so I'm guessing it's the cat then ?