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2004 Subaru OBW L.L. Bean H6. 2003 Subaru OBW L.L. Bean Former 2001 Subaru OBW VDC owner.
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Discussion Starter #1
In my 2004 LL Bean Outback I’ve changed both front O2 sensors. I tested those 2 with a volt meter, which were showing good, and I also replaced the rear O2 sensor, with a sensor that was good when it last worked in my 2001 VDC Outback. And after all this, I still have an immediate check engine light upon start up.. with the P0031 code from the scanner. what in the world do I have to do to fix this? Happy day and hello everyone!
 

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"upstream o2 sensor low input"

trace the wires/harness. look for corrosion / bad ground / fraying.

___

all the o2 sensors you bought were Densos? (no bosch/...delco )
 

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P0031 is a problem ("low") in the sensor heater circuit of the Bank 1 A/F Sensor (pre-cat).

Power for the sensor heater comes from the Main Relay. (Same power goes to the Bank 2 sensor.) The other side of the sensor heater element goes to the ECM. The ECM controls the heater, and through that, the temperature of the sensor, by switching the terminal at the ECM between high and low, where low is essentially ground. So when the heater should be off, the voltage at the ECM terminal should be "high", essentially system voltage (coming from the Main Relay). When the ECM wants the heater to be on, it grounds the terminal, which means the terminal voltage is at or near zero Volts.

The ECM OBD system continuously compares the voltage at the terminal to the logical situation of the ECM (heater on, or heater off) -- the two should be consistent. In the case of a P0031, the voltage at the terminal remains low even when the ECM control is commanding a "high".

A low can be caused by an absence of voltage from the Main Relay, an open heater element, or a discontinuity elsewhere in the circuit. It could also be caused by a short to ground in the heater control wire between the sensor and the ECM.

Troubleshooting requires checking for the required voltage, continuity, and absence of a ground.
 

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Discussion Starter #4
P0031 is a problem ("low") in the sensor heater circuit of the Bank 1 A/F Sensor (pre-cat).

Power for the sensor heater comes from the Main Relay. (Same power goes to the Bank 2 sensor.) The other side of the sensor heater element goes to the ECM. The ECM controls the heater, and through that, the temperature of the sensor, by switching the terminal at the ECM between high and low, where low is essentially ground. So when the heater should be off, the voltage at the ECM terminal should be "high", essentially system voltage (coming from the Main Relay). When the ECM wants the heater to be on, it grounds the terminal, which means the terminal voltage is at or near zero Volts.

The ECM OBD system continuously compares the voltage at the terminal to the logical situation of the ECM (heater on, or heater off) -- the two should be consistent. In the case of a P0031, the voltage at the terminal remains low even when the ECM control is commanding a "high".

A low can be caused by an absence of voltage from the Main Relay, an open heater element, or a discontinuity elsewhere in the circuit. It could also be caused by a short to ground in the heater control wire between the sensor and the ECM.

Troubleshooting requires checking for the required voltage, continuity, and absence of a ground.
@eagleeye , I did not use new sensors, I simply used different sensors, that were not throwing codes when I had them in the 01 VDC.

Now, PlainOM, I don't know how much of that came from your brain, but well done either way, and I'm sure a real Subaru technician would agree with you. I don't know if this will help you help me, but thanks for the help thus far.. I used a multimeter in testing all my sensors. I placed the negative probe to the body of the sensor with vice grips, then touched the positive probe to the terminal where the blue wire goes to. I switched my multimeter to 20 DC voltage, and heated up the O2 sensor with a torch. The reading on the multimeter quickly counted up to 1.0, which according to my YouTube teacher, means the sensor is ok. What might you say, or anyone else reading this, would you say, I actually ruled out by doing this test to my sensors? Could my sensors still be bad?

From what you wrote above, are you in any way pointing to that replacing my relay might be the solve? I can start checking wires next time I'm down there, but I'm not sure what to check between the O2 sensor and the ECM.. Not sure what's easy to get to , I guess I can just start tracing wires as far as I can, see what looks good isn't good...
https://youtu.be/dclC5mGtyBI?list=FL1jeZakywh67vswbtwOPCRg
 

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@Lverano

The A/F Sensor has four wires. Two are for the oxygen sensor element itself, and two are for the heater. The heater is part of the assembly, but electrically separate from the sensor element itself.

The P0031 relates to the heater circuit. On a Denso sensor, the heater wires from the sensor to its connector are black. (They can be seen in the video.) The blue and white wires are for the O2 sensor element. So, checking the element voltage at the blue wire (or between the blue and white wire) as suggested in the video isn't addressing the problem identified by the fault code.

As both the bank 1 and bank 2 A/F Sensor heaters, as well as the rear O2 sensor heater, are powered from the same point at the Main Relay, and as there's no trouble code for the other sensors, it's unlikely the Main Relay is problematic.

I too thought you had new sensors. If not, then check the resistance between the two black wire contacts at the bank 1 sensor connector. It should be less than 10 Ohms. (See step 8 in the attached FSM troubleshooting guide.)

If I understand correctly, you had the P0031 before changing the sensors. Although it's possible that the replacement has the same fault, it's somewhat unlikely. If the resistance of the heater is correct, then the problem causing the P0031 both with the original and replacement sensors is elsewhere in the heater circuit for the bank 1 sensor.
 

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Discussion Starter #6
@plain OM

I’ll give this reply another study later for sure, thank you. I might just fix all the other stuff I have going on with this car first, and then give all my focus to this O2 sensor issue last, so it might be a while til I update on this topic. But I will update, eventually.
 

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Discussion Starter #7
@plain OM I re-read that post. I’m new to the O2 sensor lingo, but I think you are telling me to check the resistance, (should be 10 ohms or less), in the black wires of my O2 sensors, because the culprit is the heater part of the sensor since I’m getting a P0031 code. The check I did according to my video only checked the element part of the O2 sensor.

I am not sure what A/F stands for, and when you say bank 1 and bank 2, I’m not sure what that means, but it seems like just another way of naming/talking whichever O2 sensor is up front. I do get the concept that you were explaining about the main relay.

I’ll ask @eagleeye to rename this thread O2 sensors for dummies.
 

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@plain OM I re-read that post. I’m new to the O2 sensor lingo, but I think you are telling me to check the resistance, (should be 10 ohms or less), in the black wires of my O2 sensors, because the culprit is the heater part of the sensor since I’m getting a P0031 code. The check I did according to my video only checked the element part of the O2 sensor.
That's correct. Check the resistance between the two black wires. (Disconnect the sensor connector from the engine wiring harness, find where the two black wires connect to pins in the connector, measure between those two pins.)

I am not sure what A/F stands for, and when you say bank 1 and bank 2, I’m not sure what that means, but it seems like just another way of naming/talking whichever O2 sensor is up front. I do get the concept that you were explaining about the main relay. . . .
A/F = air/fuel, referring to the ratio of air to fuel in the mix going into the engine.

The A/F sensors are the pre-cat sensor; i.e., the ones up front.

Auto convention is that Bank 1 is the right side of the engine as viewed from the driver seat. Bank 2 is the left side. Consequently, Bank 1 in a LHD car is on the passenger side.

The P0031 is a heater circuit problem for the Bank 1 A/F Sensor, so the one at the input end of the cat on the right/passenger side.
 

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"upstream o2 sensor low input"

trace the wires/harness. look for corrosion / bad ground / fraying.

___

all the o2 sensors you bought were Densos? (no bosch/...delco )
So we need to use denso? Why not bosch? So if i put bosch would be bad?
I got p0031 when tried run engine on disconnected o2 sensor. But was no difference in how engine worked. Still getting p0420 after some time. And then sometimes its not appear for weak or so. Sometimes same day.
 

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So we need to use denso? Why not bosch? So if i put bosch would be bad?
I got p0031 when tried run engine on disconnected o2 sensor. But was no difference in how engine worked. Still getting p0420 after some time. And then sometimes its not appear for weak or so. Sometimes same day.
from reading here it has something to do with the refresh rate,
and the way the bosch react, the subaru ECU sees them fine, but the data they send is not idea (or good).

so the ECU makes poor decisions or odd codes, or you never get a emissions ready on the cat.

____

the only bosch parts that maybe on your car from the factory = the special ABS controller for the VDC system.

Denso = part of toyota, and now subaru is partnered up with toyota.
 

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well ok if you say so then. :)
 

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well ok if you say so then. :)
the other thing that can happen if the ECU is getting poor data from the front one: running slightly rich or slightly lean.

and no code or anything to know.

___

and the smarter and newer the car the rear ones can have more play with the ECU.

(probably not anything in a EJ251/EJ252 though).
 

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what i can find is front denso DOX-0307 direct fit 510mm 92 euros , and rear DOX-0120 universal fit 750mm 40 euros. those model numbers right ?
 

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what i can find is front denso DOX-0307 direct fit 510mm 92 euros , and rear DOX-0120 universal fit 750mm 40 euros. those model numbers right ?
I start with rockuato, and Denso's own sites and compare to what others have.

my rockauto screen for them in the USA does not list those type of numbers. but what you have on a EU page maybe different.

I see:
DENSO 2345003 {#12567073, 22791AA00A} for Upstream.

and
DENSO 2344132 {#22690AA43A, 22690AA73A} For Downstream.

______

and these I would try to buy from someplace big that only sells autoparts. (hoping to not get knockoff fake junk).
 

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Discussion Starter #15
O2 sensor compatibility

That's correct. Check the resistance between the two black wires. (Disconnect the sensor connector from the engine wiring harness, find where the two black wires connect to pins in the connector, measure between those two pins.)



A/F = air/fuel, referring to the ratio of air to fuel in the mix going into the engine.

The A/F sensors are the pre-cat sensor; i.e., the ones up front.

Auto convention is that Bank 1 is the right side of the engine as viewed from the driver seat. Bank 2 is the left side. Consequently, Bank 1 in a LHD car is on the passenger side.

The P0031 is a heater circuit problem for the Bank 1 A/F Sensor, so the one at the input end of the cat on the right/passenger side.
I haven't gotten around to checking resistance in any of my sensors yet, too busy with work and BODY work. I have my 03 LL Bean pulled into the garage now. The check engine light is not on on that one, so I'm thinking of pulling both the O2 sensors off of it, and throwing them on the 04 LL Bean to see if the check engine light will come back on after I clear the code.

I look around the engine compartment of the 03 Bean, it's dirty, it's older than my 04, and yet I've not had any electrical problems with the car. The 04 is very clean, and I just don't see how there could be electrical issues. BUT 14 years is a long time for things to happen. I really need to rule out bad O2 sensors before I start spending time on other wiring issues.

I think this was probably gone over, but I forget, is there any fuses anywhere I should be checking?
 

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Discussion Starter #16
Update

It seems O2 sensors may not be compatible between all second generation Subaru Outbacks.

I called the dealer today to get a part number for my 03 LL Bean. I gave my VIN number and asked for an upstream O2 sensor and was given part number 22641AA03C

I then called another dealer and gave the VIN number for my 04 LL Bean and asked for the same O2 upstream sensor, and I was given the same part number, 22541AA03C.

Now hear this, I asked what year vehicles that part spanned and the parts guy says 03 and 04. I'm sure the sensors I have laying around that I threw on the 04 Bean are doing their job, but are throwing my engine light on for not being the correct sensors.

The car rides fine, and MPG seems normal. The spare sensors I have are from 2nd Gen Outbacks in the junkyard, and I'm not even sure I remember at this point in time what year and model I pulled them off. Of course I still had a few sensors from the 01 VDC laying around, so one of them might be on there. Anyway, this would I think partially explain why I'm getting the check engine light immediately.
 

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try matching what rock auto sells in Denso branded ones for the 2001-2004 H6.
 

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. . .I think this was probably gone over, but I forget, is there any fuses anywhere I should be checking?
In regard to the heater circuit, no; the same circuit powers the other sensor heater.

Doesn't this H6 have sensors on each side? If so, then perhaps try swapping from side-to-side to see if the fault code moves across, which localizes the problem to that sensor.
 

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Discussion Starter #19
Compatibility O2 sensors

try matching what rock auto sells in Denso branded ones for the 2001-2004 H6.
Rock Auto, I checked it out.

So, 2001 to 2004 H6 upstream O2 sensors are all interchangeable according to Rock Auto, and all have the same part number for upstream 22641AA03C, even though the dealer says only 03 and 04 upstream sensors for the H6 are comptible.

But guess what, according to Rock Auto parts database, no upstream O2 sensors from any H4 of any year 2001 to 2004 have the same part number as an H6 in the same years.

I'm sure I plucked some incompatible H4 O2 sensors from the junkyard and mixed them up with the ones from my VDC, and now I don't know what's what. @plain OM I am going to swap the ones from my 03 Bean to my 04 side for side and see what's what. The ones on the 03 Bean are good far as I know.
 

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Rock Auto, I checked it out.

So, 2001 to 2004 H6 upstream O2 sensors are all interchangeable according to Rock Auto, and all have the same part number for upstream 22641AA03C, even though the dealer says only 03 and 04 upstream sensors for the H6 are comptible.

But guess what, according to Rock Auto parts database, no upstream O2 sensors from any H4 of any year 2001 to 2004 have the same part number as an H6 in the same years.

I'm sure I plucked some incompatible H4 O2 sensors from the junkyard and mixed them up with the ones from my VDC, and now I don't know what's what. @plain OM I am going to swap the ones from my 03 Bean to my 04 side for side and see what's what. The ones on the 03 Bean are good far as I know.
do the ones you got from the pick and pull have stock #s on the plastic or on the wire casing?

you could look those up.
 
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