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Additives for intermittent P0420 on Toyota 3.0 V6 #5466221 06/30/20 09:49 AM
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Jarpin Offline OP
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So I just got back from a 1700 mile round trip vacation. While I was there, driving around in city traffic, CEL came on. I decided to dump a bottle of Chevron Techron in it, along with the Chevron mid grade for the drive home. 60 miles on IH-10, and code gone! I get back into town on Saturday night, and drive around town yesterday...code back again! Drive to work today on the highway 20 minutes.... code gone again! So apparently, stop and go driving triggers it, and highway cruising resets it. At this point, I'm ready to go full Scotty Kilmer and pour in a gallon of paint thinner on a half tank of gas to see if it helps dissolve any crud in the sensors or inside the CAT. Has anyone tried it?

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466223 06/30/20 09:52 AM
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SatinSilver Offline
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I’d try another round of Techron in it. Sounds like it could use a double or back to back dose.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466230 06/30/20 09:59 AM
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Astro14 Offline
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The code comes from a comparison between the front (Upstream) and rear (Downstream) O2 sensors. Additives likely won’t fix this...it’s more likely the result of old O2 sensors, or an old cat... Both of which wear out with time.

What vehicle? How many miles on it?


32 Packard 15W40
01 Volvo V70 T5 0W30 AMSOIL SS
02 Volvo V70 T5 0W30 Edge
02 Volvo V70 XC 0W40 Edge
05 MB S600 0W40 M1
16 Tundra 1794 5W30 Pennzoil Ultra
Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Astro14] #5466245 06/30/20 10:16 AM
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Jarpin Offline OP
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Originally Posted by Astro14


What vehicle? How many miles on it?


1999 Toyota Avalon. 266K (METHODICALLY & METICULOUSLY maintained by me and previous owner)

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466246 06/30/20 10:18 AM
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My GF's 2003 Highlander (with the 2.5L 4cyl that it shared with the Camry) threw this code intermittently. I was informed that it was actually an ECM programming issue related to the sensitivity being set too high for O2 sensor voltage readings. I did a temperature check (front vs rear) on her converter and voltage checks on the O2 sensors. The vehicle literally had no issues. Some people on here used spark plug anti-foulers to pull the O2 sensor part way out of the exhaust stream. In our case I would just clear the code and then it would go for months before throwing it again. A Toyota dealer can probably resolve the issue with an ECN reprogramming, but without knowing more about the issue it's hard to say for sure.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466272 06/30/20 10:58 AM
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ripcord Offline
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Astro14 is right. It's due for some new O2 sensors with that many miles, but likely is just a worn out cat.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466276 06/30/20 11:09 AM
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Astro14 Offline
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If additives don’t work (and I’m not optimistic) then I would consider the upstream O2 sensor. It’s the one that makes the engine run right anyway, and is often a 100,000 mile maintenance item.

If that doesn’t fix it, then I would try the spark plug antifouler on the downstream sensor.

In setting up a new IPD high flow exhaust on a turbo Volvo, I had the same code. I had brand new Bosch O2 sensors, upstream and downstream, so it wasn’t the sensors. I added this to the downstream sensor:

https://www.ipdusa.com/products/12093/125604-adjustable-oxygen-sensor-spacer-ipd-125604

Worked great. Code cleared and hasn’t been back since. The 90 degree bend keeps good clearance, and allows it to fit in tight spaces.


32 Packard 15W40
01 Volvo V70 T5 0W30 AMSOIL SS
02 Volvo V70 T5 0W30 Edge
02 Volvo V70 XC 0W40 Edge
05 MB S600 0W40 M1
16 Tundra 1794 5W30 Pennzoil Ultra
Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: ripcord] #5466277 06/30/20 11:11 AM
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Jarpin Offline OP
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Originally Posted by ripcord
Astro14 is right. It's due for some new O2 sensors with that many miles, but likely is just a worn out cat.


The rear one was replaced last year with a Denso OEM. Front one might be original.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466290 06/30/20 11:35 AM
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Farnsworth Offline
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Check the bellows hoses on air intake plumbing. They get cracks and cause exactly this strange behavior. On that same engine had cracks on both hose bottoms probably from bending them as they aged to change the air filter. Made the car drive a like new car practically and no sensors were bad. Took me a couple months to look for something different, and there the problem was, got new hoses from Toyota. Any disturbance in air intake after the maf sensor wreaks havoc and sensor codes are thrown. It doesn’t mean it’s the sensors but the signal they are getting is wrong. Worth a look.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Farnsworth] #5466298 06/30/20 11:51 AM
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Jarpin Offline OP
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Originally Posted by Farnsworth
Check the bellows hoses on air intake plumbing. They get cracks and cause exactly this strange behavior. On that same engine had cracks on both hose bottoms probably from bending them as they aged to change the air filter. Made the car drive a like new car practically and no sensors were bad. Took me a couple months to look for something different, and there the problem was, got new hoses from Toyota. Any disturbance in air intake after the maf sensor wreaks havoc and sensor codes are thrown. It doesn’t mean it’s the sensors but the signal they are getting is wrong. Worth a look.


You know, I did replace the VCG's last winter.... the big snorkel intake rubber was replaced with brand new two years ago, but the clamps might have slipped. Thanks.

Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466365 06/30/20 01:18 PM
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MolaKule Offline
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Originally Posted by Jarpin
So I just got back from a 1700 mile round trip vacation. While I was there, driving around in city traffic, CEL came on. I decided to dump a bottle of Chevron Techron in it, along with the Chevron mid grade for the drive home. 60 miles on IH-10, and code gone! I get back into town on Saturday night, and drive around town yesterday...code back again! Drive to work today on the highway 20 minutes.... code gone again! So apparently, stop and go driving triggers it, and highway cruising resets it. At this point, I'm ready to go full Scotty Kilmer and pour in a gallon of paint thinner on a half tank of gas to see if it helps dissolve any crud in the sensors or inside the CAT. Has anyone tried it?


Sometimes a back to back dosing of CatClean will help but this may only be a temporary fix if the sensors and Cats are on the decline.

Last edited by MolaKule; 06/30/20 01:21 PM.

"So many shows out there dumb-down the country. It's so admirable to be part of a show that wants people to think." (David Krumholtz 'Charlie Epps' of NUMB3RS)
Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466440 06/30/20 03:09 PM
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JMJNet Offline
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Most of the time, changing the sensors will eliminate the code.
IIRC, there are 2 front and 2 back sensors.
If not, then the catalytic needs to be changed.

This is based on my Brother's Sienna with 3.3L engine.


  • 2017 Toyota Sienna
  • 1995 GMC Suburban 6.5L Turbo Diesel


Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466458 06/30/20 03:50 PM
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Trav Offline
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Originally Posted by Jarpin
Originally Posted by Astro14


What vehicle? How many miles on it?


1999 Toyota Avalon. 266K (METHODICALLY & METICULOUSLY maintained by me and previous owner)


That's a lot of use on a cat. Some cars are more prone than others for P420 and IIRC Toyota is one of them, you did good getting it this far. +1 Astro, throw a extension on the rear O2.


ASE L1, Master. Deutsch Meisterbrief.
Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466462 06/30/20 03:58 PM
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Audios Offline
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Ive had luck taking it for a good ride on the highway and locking out overdrive, this keeps the revs up and increases the heat in the cats to try and burn off some of the build up in there. I did this with a double dose of Techron and the light has been gone for over a year in an 04 A4 3.0


2002 VW Golf TDI-T5 10/30
2007 Mercedes GL320 CDI-T6 5/40
2005 BMW M3 BMW-Twin Power 10/60
2003 BMW M5-M1 15/50
1998 BMW M3/4/5-M1 10/40
Re: Additives for intermittent P0420 on Toyota 3.0 V6 [Re: Jarpin] #5466526 06/30/20 05:14 PM
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newtoncd8 Offline
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In March, had P0420 on our RX330 (257,000 miles). Ran back-to-back bottles of Cataclean and Shell Premium fuel for a few tanks. Cleared check engine light manually, passed emissions testing (Colorado) and check engine light has been off for about 5,000 miles. Prior to using, read lots of mixed reviews, but so far, it has worked for our RX.


12 Jeep GC, 04 Lexus RX330, 77 MG-B
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