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P0420 code, i did remove the second air filter, related?

7K views 47 replies 14 participants last post by  cardoc 
#1 ·
Hello!

This is a perfectly well maintained 2007 Outback that showed me this afternoon a check engine and cruise light.
At home i plugged the obd2 scanner and got the P0420 code. Since it is related to the emission’s system, I wonder if has something to do with the second “filter” (I think it is a charcoal canister) I did remove almost a month ago :-/.
The car runs perfectly fine, no exhaust leak, no leak, no weird noises, excellent fuel economy good acceleration (it’s normal acceleration…)

What do you guys thing?
 
#2 ·
Do a search on this site. That code relates to a bad O2 sensor or degrading catalytic converter.
 
#9 ·
I would look more for codes rather than an app's analysis of pass/don't pass, assuming your check engine light is on. From codes you can then look at possible causes, of which there may be many for a particular code. If there are multiple codes look for ones with common causes.

One I'm unfortunately familiar with one the Ford V10s due to Cruise America sending me out in a defective vehicle. That code was for a cylinder misfire. That had at least five possible causes including bad plug, bad coil pack, bad injector, cracked block, blown head gasket. In my case the other situation was loss of coolant, so it was either a blown head gasket or cracked head. But again, combine the code with other codes and/or other situations and you can narrow it down.
 
#12 ·
You're not alone.
Of the many codes that the On Board Diagnostics can generate, the P0420 is probably the most challenging to resolve correctly.
Look at this thread: P0420 Diagnosis and perhaps also An Engineers Perspective on the P0420 Catalytic Efficiency code.
How do i know which o2 sensor is bad?
if the system "says" the catalitic is not woking properly and i read that the downstream sensor is basicaly a monitor for the catalytic so i isume the upstream (air fuel flow ratio) is the bad one, am i wrong?
 
#14 · (Edited)
yes, both of them thats why the doubting of which of the two sensors could be.
also in my list of suspect the MAF but becase since i did remove the charcoal mesh, and the car due the coronavirus was being used 1 or two times per week when the error appeared, could it be possible those fuel vapors in the manifold that Subaru mention are reaching the MAF sensor and altering its readings? i mean, if it is true the charcoal filter is needed because the vapors going back to atmosphere from the intake when the engine is off... and the car being parked for days and then short trips...

just an idea..
 
#17 ·
yes, both of them thats why the doubting of which of the two sensors could be.
I have no idea which sensor it could be, or even if it is a sensor problem.

There is no sure, quick answer, as should be evident from the P0420 Diagnosis thread. At best, detailed measurements/logging might reveal something that isn't right, and can explain the reason for the code. All the suspect parts in that long list could be replaced and still not correct the P0420 code if, for example, the catalytic converter is, in fact, bad.
 
#15 ·
taking this from one of the post you linked

discarding things

List of possibilities:

1 Fuel quality (low quality fuel a/k/a Murphy Gas)
2 Fuel quantity (filter or pump issue)
3 ignition issues (plugs, wires, coils injector flow)
4 temperature control (too high or too low)

5 vacuum leaks
6 exhaust leaks (not only an annoying noise, but it effects proper value reporting to the ECM by the AF and O2 sensors)
7 poor amperage conductivity (i.e. battery or cabling)

Sensor issues can be:

8 MAF (Mass Air Flow Meter)
9 MAP (Manifold Absolute Pressure)
10 ECT (Engine Coolant Temperature)
11 AF (Air/Fuel ratio a.k.a. Front O2 which can be a single bank 1 for 4 cyl or bank 1 or bank 2 H6)
12 O2 (This is the actual O2 behind the CAT that lets the computer know how the CAT is functioning)

Results:
1 NO, i did refill again with known and suggested fuel for this purpose, this time (a months ago i did use premium (93 although this car can run for regular, and in fact i get better mileage with regular) after that tank i used 89, then yesterday 87 again

2 NO, filter is new, pump too, both working good and with pressure
3 NO
4 NO
5 Pending to check, highly doubt it
6 Pending to check, although 3 weeks ago i did an extensive inspection underneath, and use to take photo of all the area to keep records (yes i keep a log of all parts, with parts numbers, details, descriptions and comments, maintenance and repairs or modifications done) and i didn't see any smoke mark of leak around the exhaust gaskets (the engine block is aluminum clean yet so it would have been easy to see) but i will check again later, although my mom asked me a week ago if there is a leakage in the exhaust, to notice, she are used to the H6 sound, the h4 for her is new, she even thought the car was bad because the engine noise and vibration lol
7 NO, new battery, a ground kit installed and alternator in good working orden.
8 SUSPECT
9 PENDING TO VERIFY
10 Pending to check, highly doubt it, since the reading in the dashboard and in the obd2 app match the actual readings of an infrared thermometer when measured manually.
11 SUSPECT
12 SUSPECT


i already ordered the filter box with the second "filter" carbon canister mesh thing, i got it for 30 bucks new on ebay... just to discard things


Advices please
 
#20 ·
yes, but i am waiting for the developer of the app to help me with the payment, its a great app, for 3.7, but i am havin an issue wit my account and i am wondering if i can pay directly (paypal), i want the full version to see if i get more data.

for the real time sensor data, need the car to be warm up or not?
yesterday i notice in real time sensors that there are some parameters related to the o2 sensors, some of them get alives when i accelerate.
 
#22 ·
I'm having fun doing the 420 diagnosis dance as well (YAY -_- ). Some quick things I tried:
Cleaned MAF sensor(no help)
Cleaned the intake throttle body (helped, but code still comes back every now and then)

I'm waiting on a Tactrix cable to come in (ordered it in Jan, was just canceled...sigh), because really the only way to diagnose is to capture the logs and analyse them. I know people have had issues with most BT adapters and logging (not enough bandwith across BT from what I understand), but you may be able to get "good enough" to see the problem.
 
#26 ·
I just went through the same thing with my 2008 Outback. First that app is great but the most useful is the live graph and the engine has to be completely warmed up, like in driving around for 15 minutes. Graph the rear O2 sensor voltage, and see after warming up if it stays close to 0.7V Mine was jumping up and down from .7 to .1 and back up. Showed a saw tooth pattern. I did not want to believe it and changed the 2 O2 sensors, the spark plugs etc etc, still same result. Then after a month of messing around, the car lost all acceleration when trying to pass on the highway. Changed the cat to an aftermarket one from Amazon, power is back on. For the heck of it I graphed the rear O2 sensor again, straight line at 0.7V. All check engine lights gone and cruise control is back (my commute is 40 miles and it was getting painful to constantly stay at speed)
 
#27 ·
Guys it is a little sad to see 26 posts on a P0420 code. Most codes have several possible root causes. But P0420 is simply the catalytic converter is not doing its job.
Look at code criteria for the conditions that set the code. For this one the rear O2 sensor pattern too closely match the front O2 sensor. Most code readers will have freeze frame data for conditions when the code was generated. Confirm fuel trims are near 0 and that there are no other codes. Then save up for a catalytic converter.
If you have a diag tool with 2-way communication you can look at both up and downstream O2 sensor live data to confirm.
 
#28 ·
Guys it is a little sad to see 26 posts on a P0420 code. Most codes have several possible root causes. But P0420 is simply the catalytic converter is not doing its job.
Look at code criteria for the conditions that set the code. For this one the rear O2 sensor pattern too closely match the front O2 sensor. Most code readers will have freeze frame data for conditions when the code was generated. Confirm fuel trims are near 0 and that there are no other codes. Then save up for a catalytic converter.
If you have a diag tool with 2-way communication you can look at both up and downstream O2 sensor live data to confirm.
There is a topic on this forum about the P0420 code that is over 2000 posts long and I believe that topic might prove your comments to be a little too narrow.

The reason that topic is so long is because replacing the catalytic converter does not always solve the P0420 code and/or people who just replace the catalytic converter without troubleshooting the actual cause of the problem, end up with failed catalytic converter a second time.

Tread your own path but beware!

Seagrass
 
#29 ·
Fair challenge and the reason for confirming there are no other codes or performance issues. If you run with a dead misfire due to ignition failure then the raw fuel will grenade the cat. Then hopefully the misfire and rich code are first repaired before replacing the cat. Multiple codes? Better investigate and fix the cause. But this code is a measure of cat efficiency and tells you 1 thing. Much different compared to a rich, lean or misfire code - all with multiple sources of root cause.
 
#32 ·
Honestly I didn't realize we HAD a 2nd air filter, had to goto the parts diagramto see what they were talking about. Been running for a while without it. really don't think its that. We just may be hitting the age where the cats have gotten too clogged/used (and are seeing more of these codes pop up)

Out of the 3 cars I've dealt with that had a p0420, all 2 where caused by the rear 02 sensor or front 02 sensor not reading... so it's not always a Cat, if the sensor isn't reporting correctly, it can throw p0420.
The code is the first to be thrown if there is a discrepancy between what the sensors say and what is actually happening with the A/F mixture (probably because it has the tightest tolerance, and is the first to throw a code if something isn't dialed in). More than likely it's either a bad cat or a bad o2 sensor, BUT there's a whole list of issues that can contribute. The p0420 thread in the problems and maintenance section the first post is well thought out and shows just how much interplay there is in the car emissions calculations that the CPU does.
 
#34 ·
P0420 is most commonly the O2 sensors. They aren’t that expensive online and are relatively easy to replace. You can usually borrow the special socket, (it has a cut-out to accommodate the wire) from an auto parts store. If is the CAT, you don’t need the OEM replacement. Volvo wanted over $2,000 for an S40 catalytic converter. The parts guy sent me to a local custom exhaust shop that charged me $225 installed.

Yes, there are many possible P0420 causes. But focus on the two most common, O2 and CAT. Keep it simple.
 
#36 ·
Thanks to all you guys.
Today i just receive the o2 sensor i ordered, i will wait for the charcoal filter to install the filter first.

i was able to solve the app for getting live view of the sensors, i think the graphic view is the best

i got here 4 screen captures
(all of the them the car is properly warmed up)
1-the car is at idle
2-accelerating from a stop light
3-cruising at about 50mph
4-cruising
5-slowing down
481746
 
#37 ·
Graph only the 2 oxygen sensor voltage alone so you can see the patterns of both. If they look identical then the catalyst is not doing anything- the reason for the p0420 code.
The P0420 code is not related to the charcoal canister you removed. Removing the charcoal canister should have created some leak codes for evaporative emissions.
 
#39 ·
Charcoal CANISTER is a different thing than what I thought you were talking about.

You were talking about the "2nd filter" shown here: AIR CLEANER & ELEMENT. AIR CLEANER.. Subaru | Subaru Parts Online between the air filter and the upper case correct?

The charcoal canister is connected to your fuel system to help keep pressure from building up in your fuel tank. see: FUEL PIPING. FILLER PIPE.. Subaru | Subaru Parts Online and would be located under the car (99% sure its under the car....never really had a need to put eyes on it)
 
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