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Old 08-12-2020, 09:38 PM   #1
mjm4jc
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Default O2 sensor voltage.

Hello again,

I have an 09 NA sedan that has been throwing a P0420 code for years now. I got tired of resetting it with my scanner and thought I'd finally look into it. the car runs great with plenty of power, so I wasn't too quick to assume a bad cat. So I hooked up my scanner to see what my O2 sensors are doing in the live data. Before I get into the voltage values, the first issue is that I cannot get an answer in my searches whether my pre-cat sensor is an O2 sensor or an AFR sensor. I read somewhere that Subaru started using AFR sensors, but could not find out what years.

Anyway, the voltage of the pre-cat sensor is pretty steady at around 2 to 2.4v. If it is an O2 sensor, then this is way out of range. From what I read the pre-cat O2 sensor should be between .1v and .9v.

The post cat sensor, fluctuates between 0 and .7v. It changes these values pretty quick. I think that this range is ok, except for the fact that there are a few times when it goes to flat zero.

Anyway, just trying to rule out (or rule in) O2 sensors before I do any more troubleshooting. I have no other codes with this vehicle besides the P0420. Any insight you can offer is much appreciated.

Thanks!
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Old 08-12-2020, 10:46 PM   #2
serrilion
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Normaly its the Rear 02 Sensor acting up, the wires could have been nicked or it just faulted out or needs a riser for a cheap fix.


a decent video if your looking to cheap it out. His values show like yours fluctuating and then once installed they level out. Hope this helps
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Old 08-14-2020, 07:15 PM   #3
mjm4jc
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Thank you Serrilion. At the moment, I'm looking for a solution and not real concerned about making the light go out. What I really need is for someone to confirm what their front O2 sensor voltage reads in live data with car fully warm. If I know that the front O2 sensor is working properly, then I can start looking at the rear O2 and the cat.

Can anyone confirm their pre-cat O2 sensor voltage for me? Mine was around a steady 2v slightly fluctuating between 1.9--2.4v.

Thanks
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Old 08-18-2020, 06:43 PM   #4
Elbert Bass
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Quote:
Originally Posted by mjm4jc View Post
Thank you Serrilion. At the moment, I'm looking for a solution and not real concerned about making the light go out. What I really need is for someone to confirm what their front O2 sensor voltage reads in live data with car fully warm. If I know that the front O2 sensor is working properly, then I can start looking at the rear O2 and the cat.

Can anyone confirm their pre-cat O2 sensor voltage for me? Mine was around a steady 2v slightly fluctuating between 1.9--2.4v.

Thanks
Your pre-cat "O2" sensor is a broadband Air/Fuel Ratio sensor. Subaru has used A/F sensors in the front position since 2000.
With A/F sensors voltage is not a definitive indication. A/F sensor data is determined by current draw. Rich is negative, lean is positive. It can also be displayed as a Lambda value of +/- 1.00, above 1 is lean below 1 is rich.
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Old 08-18-2020, 10:13 PM   #5
mjm4jc
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Thank you for confirming that E.B. I suspected that the front sensor was an AFR sensor. From other sources I read, 2.5v is good after fully warm. But like you said, the voltage itself is not the litmus test. The Lambda is normally .99 to 1.1 roughly. So it's not that rich or lean at least what's coming into the cat.
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Old 08-18-2020, 10:15 PM   #6
mjm4jc
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Should the rear O2 sensor being bouncing around real fast from 0 to .8 volts? My understanding was that it should be steady at around .5 v but some fluctuation is ok.
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Old 08-20-2020, 10:37 AM   #7
Elbert Bass
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Quote:
Originally Posted by mjm4jc View Post
Should the rear O2 sensor being bouncing around real fast from 0 to .8 volts? My understanding was that it should be steady at around .5 v but some fluctuation is ok.
Yes and no. Depends on context/conditions.
How Subaru has us test P0420 is inspect for exhaust leaks anywhere between the engine and after the o2 sensor then to warm up the vehicle and perform a drive cycle while recording throttle position, front A/F, and rear O2. There are other parameters like road speed and engine temp but those are just to verify we did the test correctly when submitting to Subaru on warranty claims.
Drive the vehicle on flat highway at a steady throttle and steady speed between 50-65 MPH for a couple minutes then release the throttle and let the car slow down for 5-10 seconds. Do it a couple times.
During steady driving if the A/F is steady and O2 sensor swings value rather quickly back and forth it's the cat. When you release the throttle The A/F reading should quickly and sharply go full lean followed by the O2 sensor. If the O2 goes lean first the A/F sensor is slow/bad.
O2 sensors are more straight forward either the heater has failed and will show an open heater circuit or will have poor response.
What I have found in performing too many of these test over eight years as there is a bit of interpretation of the data, especially in determining sensor issues.
An engine with a persistent long term rich condition will fail both sensors and the catalyst. A cheap catalyst will fail very quickly under those conditions or due to a faulty A/F sensor.
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Old 08-21-2020, 08:42 PM   #8
mjm4jc
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Quote:
Originally Posted by Elbert Bass View Post
Yes and no. Depends on context/conditions.
How Subaru has us test P0420 is inspect for exhaust leaks anywhere between the engine and after the o2 sensor then to warm up the vehicle and perform a drive cycle while recording throttle position, front A/F, and rear O2. There are other parameters like road speed and engine temp but those are just to verify we did the test correctly when submitting to Subaru on warranty claims.
Drive the vehicle on flat highway at a steady throttle and steady speed between 50-65 MPH for a couple minutes then release the throttle and let the car slow down for 5-10 seconds. Do it a couple times.
During steady driving if the A/F is steady and O2 sensor swings value rather quickly back and forth it's the cat. When you release the throttle The A/F reading should quickly and sharply go full lean followed by the O2 sensor. If the O2 goes lean first the A/F sensor is slow/bad.
O2 sensors are more straight forward either the heater has failed and will show an open heater circuit or will have poor response.
What I have found in performing too many of these test over eight years as there is a bit of interpretation of the data, especially in determining sensor issues.
An engine with a persistent long term rich condition will fail both sensors and the catalyst. A cheap catalyst will fail very quickly under those conditions or due to a faulty A/F sensor.


That is an excellent explanation E.B. I will give that a try on Sunday.
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Old 08-21-2020, 08:49 PM   #9
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Since this question is related to my original post, I'll ask it here. I'm a currently changing spark plugs, wires, valve cover gaskets and grommets to rule out they having anything to do with the P0420 code. I could not believe the amount of oil that was on the plugs/wires. Anyway, as you know, to get the driver side valve cover up and out, you pretty much have to remove the fuel rail. With this said, the main fuel line that feeds that fuel rail needs to come off to get the rail out of the way. The blue lock on the fuel line is holding me up big time. Can't seem to get it off. And even if i can get the fuel rail up and out of the way, I still can't see how that valve cover is going to come out when there is no clearance by the lower rear corner where it's wedged up against the frame rail. Any suggestions?
Thanks!
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Old 08-21-2020, 09:28 PM   #10
mjm4jc
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Update: Got the blue locking clip off and rail is out of the way. I still cannot figure out how to get the valve cover (driver side) out. Whatever the trick is, I'm not getting it. I tried rotating it all sorts of ways. To think that Subaru couldn't have engineered a dimple in the frame rail or something to get this out with ease, like any other valve cover I've ever done, is disappointing. Anyway, if you know what the trick is, let me know.
Thanks!
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Old 08-21-2020, 10:08 PM   #11
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So after thinking about it a bit more, it seems that to just replace the gasket and the rubber grommets, I don't really need to remove the valve cover. I can maneuver it enough (with all of the bolts out), so that I should be able to get the gasket on and grommets. If I were doing a valve adjustment, then I would consider removing it completely.
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Old 08-22-2020, 12:41 PM   #12
Elbert Bass
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Pull the nuts off the motor mounts under the crossmember and you can jack the engine up a little bit by lifting under the front diff. then you can shift the engine side to side to give you room. Might as well adjust the valves while you're there.
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Old 08-30-2020, 11:10 AM   #13
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Don't mean to sound rude but if he is having trouble taking the valve cover off, he is going to have a hell of a time trying to adjust valves with the engine/heads in the car. Not necessary imo.
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Old 08-31-2020, 10:55 AM   #14
Elbert Bass
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Originally Posted by blue wrench View Post
Don't mean to sound rude but if he is having trouble taking the valve cover off, he is going to have a hell of a time trying to adjust valves with the engine/heads in the car. Not necessary imo.
That's why he is here asking for help. Don't know how old you are but before there was internet and chat groups you were SOL for help unless you had a friend that was a mechanic. You bought a Haynes manual and made mistakes until you figured it out.
You don't have to participate here if you don't care to help someone out that may not know as much as you.
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Old 08-31-2020, 10:01 PM   #15
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That's why he is here asking for help. Don't know how old you are but before there was internet and chat groups you were SOL for help unless you had a friend that was a mechanic. You bought a Haynes manual and made mistakes until you figured it out.
You don't have to participate here if you don't care to help someone out that may not know as much as you.
I'm not sure what part of my post specifically made you think that I do not care. I realized as I typed it that what I said could be taken the wrong way, hence the "I do not mean to sound rude" part. But I admit I could have added more context.

But I will also say that you could have added more aswell, as far as the valve adjustment "while you're in there". You seem like you might have worked on a subaru engine or two before, so I would imagine that while you are probaby comfortable doing a valve adjustment in the car now it was probably not very fun the first time you did it with the frame rail right next to the head, using all 3 hands to play with the feeler guage, flat head and wrench. You already know that there are a number of things that could go wrong, and we don't know if this is his only car either. Better idea would be to find a junk head to practice on first.
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Old 09-02-2020, 11:14 AM   #16
Elbert Bass
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Quote:
Originally Posted by blue wrench View Post
I'm not sure what part of my post specifically made you think that I do not care. I realized as I typed it that what I said could be taken the wrong way, hence the "I do not mean to sound rude" part. But I admit I could have added more context.

But I will also say that you could have added more aswell, as far as the valve adjustment "while you're in there". You seem like you might have worked on a subaru engine or two before, so I would imagine that while you are probaby comfortable doing a valve adjustment in the car now it was probably not very fun the first time you did it with the frame rail right next to the head, using all 3 hands to play with the feeler guage, flat head and wrench. You already know that there are a number of things that could go wrong, and we don't know if this is his only car either. Better idea would be to find a junk head to practice on first.
Hence the professional recommendation to loosen the motor mounts and jack the engine up.
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