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P0420 Code; Which O2 sensor do I replace?


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I installed an invidia Catted Jpipe and had a PDX Protune done about 2 months ago.

 

I ran into a code that occurred 3 times now when driving on the freeway only at around 60-70 mph which was a P0420 code.

 

I have been told by fellow Subaru enthusiasts in my area to replace the front O2 sensor, but read on this forum that it should be the rear sensor.

 

On the 2011 LGT, the O2 sensors are close to each other so I really dont know which should be the one to replace now due to different opinions.

 

Any help appreciated.

Kevin

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That is the most normal code that is disabled when going stage 2. Even the cobb stage 2 ots tune disables it. The code essentially means that the exhaust coming out of the cat is too hot(or something like that). Which if you're using a high flow cat, makes perfect sense since it doesnt work as well as the stock one.

 

This is why people use a pair of spark plug anti-foulers to move the sensor out of the direct exhaust stream. Some downpipes come with a donger thing to effectively do the same thing.

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Why would a tuner just blindly disable a code when a CAT is still being used. Mine isn't disabled either, nor would I want it to be.

 

P0420 is a cat efficiency code. an OE cat will stabilize o2 measurement while an aftermarket hi flow cat might not and trigger the code. it depends on the cat. some are better than others, but in general its an unnecessary code since the aftermarket cat may be working, but not up to the specs the O2 sensor checls for and you'll get nuisance codes thrown. this is why most stage 2 maps disable the O2 check for that code.

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It would be the rear sensor, but what whitetiger said is accurate.

 

When I had my stock cat, my stage 1 tune threw P0420 but the stock tune was fine. Now that I have the invidia catted downpipe, its not even effective enough to handle the stock tune's fueling, so I will permanently have P0420 no matter which tune I'm on (unless the code is disabled).

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Thank you everyone for the advice and suggestions.

 

I sent an email to my tuner and he replied the next day with the modified protune stage 2 map for my car's mods.

 

I will see if I throw the same code or not in the future to verify if the new protune map will solve the problem.

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if the code is disabled, the emission monitors will still report as ready. I passed an OBD2 state inspection 3 times with no cat at all. he will pass.

 

 

What software did you use Open Source? I had the cobb AP and was not able to mass inspection as the cat readiness would not ready with the P0420 turned off.

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Also, every state is different for what they allow for number of sensors not ready to determine if they pass you or not. Mass used to be 2 sensors allowed to not ready.... so I used to pass all time without issue.

 

In the last 3 years.... not so much.

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What software did you use Open Source? I had the cobb AP and was not able to mass inspection as the cat readiness would not ready with the P0420 turned off.

 

i had a cobb AP v1. You still have to do a drive cycle to get all the monitors ready. that doesn't change. i used the torque app / BT OBD2 reader to view the monitors to ensure i had properly completed the drive cycle. stopping the p0420 code should not affect the cat readies monitor from being ready once a drive cycle is complete.

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i had a cobb AP v1. You still have to do a drive cycle to get all the monitors ready. that doesn't change. i used the torque app / BT OBD2 reader to view the monitors to ensure i had properly completed the drive cycle. stopping the p0420 code should not affect the cat readies monitor from being ready once a drive cycle is complete.

 

This. You could technically, if you wanted to, force readiness with open source software. But really you shouldn't need to.

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just to clarify, do you know what i mean by drive cycle? there is a specific series of actions one must do to complete a drive cycle to make all the monitors ready. if you have access to an BT OBD2 dongle, load up an app like torque to see the live monitors. in some cases, based on people routs and driving habits, its possible to drive hundreds of miles without completing the proper drive cycle.
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Interesting, I've had no problems with open source tunes in my cars

 

You won't with Open Source. I have cobb and started having issues starting a few years ago.

 

I'm not trying to turn this into a debate. I'm well aware what tuning software does what.... I am just trying to share my experience that I have had with a certain states inspection software.

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just to clarify, do you know what i mean by drive cycle? there is a specific series of actions one must do to complete a drive cycle to make all the monitors ready. if you have access to an BT OBD2 dongle, load up an app like torque to see the live monitors. in some cases, based on people routs and driving habits, its possible to drive hundreds of miles without completing the proper drive cycle.

 

Yes I am familiar. I have a few code readers that specifically do readiness tests. I'm telling you in Mass they changed what the emissions computer was allowing or looking for. In the past Mass would allow 2 sensors not to be ready for my wrx. Well they got smart and changed it to only allow 1 not to ready.

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I found this.... I will give it another shot.

 

Subaru recommends the following conditions and drive cycle to get the monitors on its vehicles completed in the shortest amount of time:

 

1. Make sure the fuel tank is between one-quarter and three-quarters full; half a tank is ideal.

 

2. Start the engine and immediately drive the vehicle for at least 15 minutes at a speed greater than 50 mph. During the warmup, try to avoid sudden acceleration, hard braking and/or lane changes.

 

3. Drive the vehicle at a steady 55 mph for three more minutes. During this stage of the drive cycle, it's crucial that you keep throttle angle changes to a minimum.

 

4. Bring the vehicle hack to the shop, hook up the scan tool and look at the status of the readiness monitors. Repeat the drive cycle, if necessary.

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I found this.... I will give it another shot.

 

Subaru recommends the following conditions and drive cycle to get the monitors on its vehicles completed in the shortest amount of time:

 

1. Make sure the fuel tank is between one-quarter and three-quarters full; half a tank is ideal.

 

2. Start the engine and immediately drive the vehicle for at least 15 minutes at a speed greater than 50 mph. During the warmup, try to avoid sudden acceleration, hard braking and/or lane changes.

 

3. Drive the vehicle at a steady 55 mph for three more minutes. During this stage of the drive cycle, it's crucial that you keep throttle angle changes to a minimum.

 

4. Bring the vehicle hack to the shop, hook up the scan tool and look at the status of the readiness monitors. Repeat the drive cycle, if necessary.

 

exactly. do this.

 

Yes I am familiar. I have a few code readers that specifically do readiness tests. I'm telling you in Mass they changed what the emissions computer was allowing or looking for. In the past Mass would allow 2 sensors not to be ready for my wrx. Well they got smart and changed it to only allow 1 not to ready.

 

the point i'm making, is that, at least with a AP tune properly disabling the p0420 code, that all sensors/monitors would be ready with a properly completed drive cycle. i'd never go to a NJ state inspection with 1 sensor not ready even though i had no cats at all.

 

Other codes related to the downstream sensor such as p0137 or sensor heater circuit codes could still occur and make a monitor not ready.

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i had a cobb AP v1. You still have to do a drive cycle to get all the monitors ready. that doesn't change. i used the torque app / BT OBD2 reader to view the monitors to ensure i had properly completed the drive cycle. stopping the p0420 code should not affect the cat readies monitor from being ready once a drive cycle is complete.

 

This wasn't the case for me. I drove over 1200 miles trying to get it to set, a good portion of which doing the exact drive cycle prescribed by subaru, and it wouldn't complete. I flashed my stock tune back on and it completed within 5 minutes. In PA you have to have all monitors passed, so I ended up two months overdue on my inspection messing with it.

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exactly. do this.

 

 

 

the point i'm making, is that, at least with a AP tune properly disabling the p0420 code, that all sensors/monitors would be ready with a properly completed drive cycle. i'd never go to a NJ state inspection with 1 sensor not ready even though i had no cats at all.

 

Other codes related to the downstream sensor such as p0137 or sensor heater circuit codes could still occur and make a monitor not ready.

 

I would have to go back and look at the inspection sheet they print out to see what sensor were not ready and un-supported. I may have thrown them out tho.

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This wasn't the case for me. I drove over 1200 miles trying to get it to set, a good portion of which doing the exact drive cycle prescribed by subaru, and it wouldn't complete. I flashed my stock tune back on and it completed within 5 minutes. In PA you have to have all monitors passed, so I ended up two months overdue on my inspection messing with it.

 

weird. for me, they would set like clockwork after a drive cycle.

 

Same goes for my MX-5 now. have an ecutek tune on it with just 1 Hi-flow cat instead of the 2 OE ones, and i can get all the monitors ready in no time. been through state inspection twice. :iam:

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