P0420 Diagnosis - Page 157 - Subaru Outback - Subaru Outback Forums
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post #1561 of 1609 (permalink) Old 03-26-2019, 08:02 PM
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One more question for Cardoc; I took my old cat and relieved it of all the catalyst honeycomb material. I will bolt this in place of my current cat for testing purposes. My question is, should I test with the AF and O2 sensors installed (since I have the option) or will the test with the sensors uninstalled (and holes plugged) be a better diagnostic for the purpose of seeing what knock correction does?

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post #1562 of 1609 (permalink) Old 03-27-2019, 08:35 AM Thread Starter
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If you are gutting the CAT, you would want the sensors installed. The AF sensor determines fueling based on what comes out of the heads prior to the CAT. The rear one determines CAT function. If you leave it unplugged, you'll get a circuit code. If you plug it in, you get a 420 code. But mainly, the goal is to determine if restriction is causing the knock correction, so minimally have the AF sensor connected and installed.



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post #1563 of 1609 (permalink) Old 03-27-2019, 12:35 PM
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Thanks Cardoc. I'll be trying it this weekend since fixing the exhaust leak wasn't the cure.
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post #1564 of 1609 (permalink) Old 03-27-2019, 07:28 PM
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So glad I found this thread

CARDOC! I've been battling a 420 on my 2000 Legacy for 2 years now. I've replaced fuel pressure regulator, PCV, air filters, plugs and wires, and the fuel filter. I probably have forgotten about some. I have not replaced the thermostat and do not think it is the culprit as I live in Canada and my life depends on it.

I may have a slight exhaust leak before the cat, but I've tried to make it as small as possible. I doubt its worse than factory however.

Heres the story. My car will rarely just shut off at red lights. The idle will drop low and once I am about to accelerate, my engine will bog slightly and die. It sputters slightly when I start it back up, and it will sometimes throw a Flashing AT Oil Temp light with Code 45 (Intake manifold pressure signal Detects open or shorted input signal circuit.)

I have had it looked at and it seems to be a problem with my fuel trims. I usually get Code 45 along with CEL 420 and rarely 0107. While those happen every once in a while, the 420 happens almost constantly. My fuel economy seems like its worse too.

I do not have a fancy way to check codes, but I do have a very basic code reader. My STFT when driving ranges from 3.0 to -0.8 depending on if I'm on the gas. BUT my LTFT is -20 when I'm off the gas, and -12ish when I'm accelerating. I know you usually require more information, and I'm willing to get you that if you can tell me how to get it.

Thank you so much. Any help is appreciated people.
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post #1565 of 1609 (permalink) Old 03-28-2019, 09:02 AM Thread Starter
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CARDOC! I've been battling a 420 on my 2000 Legacy for 2 years now. I've replaced fuel pressure regulator, PCV, air filters, plugs and wires, and the fuel filter. I probably have forgotten about some. I have not replaced the thermostat and do not think it is the culprit as I live in Canada and my life depends on it.

I may have a slight exhaust leak before the cat, but I've tried to make it as small as possible. I doubt its worse than factory however.

Heres the story. My car will rarely just shut off at red lights. The idle will drop low and once I am about to accelerate, my engine will bog slightly and die. It sputters slightly when I start it back up, and it will sometimes throw a Flashing AT Oil Temp light with Code 45 (Intake manifold pressure signal Detects open or shorted input signal circuit.)

I have had it looked at and it seems to be a problem with my fuel trims. I usually get Code 45 along with CEL 420 and rarely 0107. While those happen every once in a while, the 420 happens almost constantly. My fuel economy seems like its worse too.

I do not have a fancy way to check codes, but I do have a very basic code reader. My STFT when driving ranges from 3.0 to -0.8 depending on if I'm on the gas. BUT my LTFT is -20 when I'm off the gas, and -12ish when I'm accelerating. I know you usually require more information, and I'm willing to get you that if you can tell me how to get it.

Thank you so much. Any help is appreciated people.
Datalogging is done with Rom Raider and a Vag-COM KKL 409.1 cable. You then upload the data file in a zip folder to a post where it can be downloaded and viewed.

The MAP sensor code is hint. The MAP tells the ECM what the volume of air is coming in to the intake. It uses the MAP, IAT and Throttle position to calculate air mass and then it fuels the car based on this feedback. If there is an issue with your MAP sensor, it throws things off. An exhaust leak in front of the front AF sensor will throw things off.

Since the ECM is reducing fuel, that means that the fuel map the ECM uses is having to be altered to maintain an average of 14.7:1 ratio or 1:1 Lambda. 20% decrease means the ECM is seeing a rich condition in the exhaust, usually unburned fuel. This could be due to a leaking injector, restriction in the exhaust, misfires, or the metering feedback from sensors is incorrect and the ECM is actually leaning out the combustion erroneously.

I would start with the MAP sensor. It is probably clogged up with carbon and sending a low pressure reading which would make the ECM add fuel, then reduce the fuel based on the AF feedback. You also have a barometer on the strut tower that may be giving false information and the ECM doesn't see it as a problem. Sometimes you get a MAP/BARO correlation code, sometimes not. You have to be able to see what the computer sees as current atmospheric pressure and compare that to the map sensor with the engine off, then calculate vacuum with the engine idling in comparison to the barometer. You could also have a bad temp sensor. The thermostat may work, but if the temp sensor is sending low temp readings back, the ECM adds fuel to warm it up. But when combustion temps are high enough to burn efficiently, the fuel has to be reduced because is comes out of the head in the rich zone.

Data logging would help.


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post #1566 of 1609 (permalink) Old 03-28-2019, 09:28 AM
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Sounds good! I’ll get my hands on those. It could take me a while, but I’ll be back. Thank you for your help, you have no idea how glad I am to see that you’ve been an active user for so long!
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post #1567 of 1609 (permalink) Old 03-28-2019, 11:32 AM
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Sounds good! I€™️ll get my hands on those. It could take me a while, but I€™️ll be back. Thank you for your help, you have no idea how glad I am to see that you€™️ve been an active user for so long!
I am new to this forum so hopefully this reply works... Cardoc, is there a way to clean the MAP sensor?
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post #1568 of 1609 (permalink) Old 03-28-2019, 01:12 PM Thread Starter
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I am new to this forum so hopefully this reply works... Cardoc, is there a way to clean the MAP sensor?
take it off and spray it down with throttle body cleaner or B-12 to loosen up any carbon that may be in the orifice

if the MAP is not clogged, and you'll see it, then there may be an issue with electrical conductance or your battery

I've never had a MAP sensor fail from age


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post #1569 of 1609 (permalink) Old 03-29-2019, 03:21 PM
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I am new to this forum so hopefully this reply works... Cardoc, is there a way to clean the MAP sensor?
take it off and spray it down with throttle body cleaner or B-12 to loosen up any carbon that may be in the orifice

if the MAP is not clogged, and you'll see it, then there may be an issue with electrical conductance or your battery

I've never had a MAP sensor fail from age
Map Sensor had fair amount of oil on it, but not tons. Its a very old engine and there could be a bit of blow by. I make sure to change my pvc often though. I will download RomRaider tonight and try using a cord to link my OBD2 reader to my laptop. It may work. If not, I will have to order one of those that you recommended, and Ill send you the data. Where is the temp sensor located? because thats cheap and very possible to be a culprit.

Aside, I have had misfires before from a failed plug wire. I dont know if that could have caused other issues but yeah.. Thank you! Enjoy the weekend!
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post #1570 of 1609 (permalink) Old 03-31-2019, 08:24 AM Thread Starter
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Map Sensor had fair amount of oil on it, but not tons. Its a very old engine and there could be a bit of blow by. I make sure to change my pvc often though. I will download RomRaider tonight and try using a cord to link my OBD2 reader to my laptop. It may work. If not, I will have to order one of those that you recommended, and Ill send you the data. Where is the temp sensor located? because thats cheap and very possible to be a culprit.

Aside, I have had misfires before from a failed plug wire. I dont know if that could have caused other issues but yeah.. Thank you! Enjoy the weekend!
The temp sensor is on the water pipe under the intake behind the power steering pump.

Misfires can also be caused from a leak on the coil pack tower. You would look for cracks in the tower. Also look at the ignition cable ends for signs of carbon tracing.



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