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'18 Tacoma, '16 Legacy, '04 Outback, ''73 MGB Roadster
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Discussion Starter #1
My daughter's '05 threw a P0420 code, catalyst efficiency too low, or something like that. About 80k miles, running great. It actually showed two codes, both the same apparently. I reset them with my scanner a few hours ago, and so far they've not come back on.

I'm thinking that it's the downstream O2 sensor(s) setting the code, and was wondering if it could be a dirty, clogged, or broken cat causing the sensor to throw the code. It's happened in both of our '96 Jeeps, with broken/rattling cats, and a simple reset has kept it away for months now.

I'm rather unfamiliar with the O2 sensors in either our '04 OBW or my daughter's '05, as I've never had to chase problems with these sensors yet, and I know our '04 has two pre-cats and a main one, but the '05 exhaust is a little different, ending in duals. How many downstream sensors are there? Of course the ground is wet today, and it's dark, so I didn't get a chance to get under it to look around. I'm hoping the reset will last until I get a chance to get under it and examine it more thoroughly. Sorry about the length of my rant, and thanks for looking!
 

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OBW H6 VDC, Tribeca, XT6
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For 2005's check into the factory recalls, TSB's, and issues regarding reprogramming the ECU - for 2005-ish vehicles only. Do a google search for 2005's. That particular issue can cause problems, so make sure your vehicle doesn't need the recall done to the ECU.

You can drive the car forever with the P0420 code, it is completely benign for driving and has no effect in engine performance itself. The rear O2 sensor isn't even used by the engine ECU, you can unplug and remove those rear O2 sensors and the car will drive exactly the same. In my state of WV there are no emissions so many folks just ignore it and run the cars forever that way. Doesn't matter. I say all that so you know it's not an emergency - with the caveat being the 2005 comments I first made.

In general though this code is a debacle and time and money sucker. This code is extremely common and happens all the time, even on other manufacturers. It is rather random - the code will return in a day or months or a year. Simple procedure for a home DIY person or someone like me that sees this all the time:

1. make sure there are no leaks
2. replace the rear O2 sensor
3. replace the front O2 sensors
4. install the $5 extenders and never think about it again

In general I would do items 2 and 3 as I plan to keep the cars for a long time and having new O2 sensors is a good thing over the course of another 100,000+ miles anyway. But those steps can be skipped if you don't want to pay for the expense right now and have no other issues (like bad gas mileage).

My friend just had it come up on her 250,000 mile honda that has other issues - no need for O2 sensor expenses on that vehicle - so I skip straight to step #4 for $5.

do not replace the catalytic converter, that's one of the biggest jokes in shops/dealers all over the country right now. thousands of people are paying mad bucks to replace expensive converters that pass emissions. stupid.

of course there are anecdotal outliers - like has the car been wrecked such that the O2 wiring might be compromised or has the engine ever had catastrophic failure or work such that the converter may have been contaminated and damaged....but you haven't indicated anything and those are statistically unlikely so we don't assume those to be the case.
 

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'18 Tacoma, '16 Legacy, '04 Outback, ''73 MGB Roadster
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126 Posts
Discussion Starter #4
Thanks for the replies! I reset the code, and so far, I've not heard back that it's reappeared. Like I said it has come up on our two '96 Cherokees, both of which do have broken innards on the cats that rattle, and one reset seems to have cured the light. I know that the downstream O2 sensor merely monitors the operation of the cat, and does not control the fuel management of the engine. I think that soon I'll install the Ultragauge on it for a while to monitor the closed loop operation of the engine, to make sure the main O2 sensor(s) are still doing the job.
 

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A ScanGauge II will spit out a defective O2 sensor code, but other than that, you can't monitor sensors as you can with an OBDII code checker, to the best of my knowledge. What the ScanGauge II will do is reset a code on the fly.

The UltraGauge may not (won't?) allow the reset of codes while the vehicle is moving. The designers are control freaks, and they disable data acquisition if you press any buttons to change the display while the vehicle is moving. I find this incredibly aggravating that they will try to control how I use the device. Shame on them.

I use the ScanGauge II to reset that P0420 code when it comes on as I drive, and the Check Engine Light may thereafter stay off through several subsequent engine start cycles.

Changing the rear (or the front) O2 sensor is not at all likely to fix a P0420 code. If the sensor was at fault, it's code would come up - so don't waste your money on a new sensor, for this reason. My O2 sensors (made by Denso) still function perfectly after 215k. That P0420 code has been coming back on since about 80k, and that's about how long I have been resetting it with a ScanGauge l and ll as I drive.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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A ScanGauge II will spit out a defective O2 sensor code, but other than that, you can't monitor sensors as you can with an OBDII code checker, to the best of my knowledge. What the ScanGauge II will do is reset a code on the fly.

The UltraGauge may not (won't?) allow the reset of codes while the vehicle is moving. The designers are control freaks, and they disable data acquisition if you press any buttons to change the display while the vehicle is moving. I find this incredibly aggravating that they will try to control how I use the device. Shame on them.

I use the ScanGauge II to reset that P0420 code when it comes on as I drive, and the Check Engine Light may thereafter stay off through several subsequent engine start cycles.

Changing the rear (or the front) O2 sensor is not at all likely to fix a P0420 code. If the sensor was at fault, it's code would come up - so don't waste your money on a new sensor, for this reason. My O2 sensors (made by Denso) still function perfectly after 215k. That P0420 code has been coming back on since about 80k, and that's about how long I have been resetting it with a ScanGauge l and ll as I drive.
Maybe you should start looking into what is wrong with the engine performance that the code keeps popping up.

http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html

And: http://www.subaruoutback.org/forums...spective-p0420-catalytic-efficiency-code.html
 
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