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Discussion Starter #1
disconnected harness behind glovebox, also P0037 code

Hi new member here, owner of 2006 Outback 2.5i Limited. I am a refugee from LegacyGT-dot-com, where there don't seem to be a lot of people active with this generation.

In the process of diagnosing a P0037 code (how I came across this forum), I noticed a connector, just under the top edge of the carpet behind the glovebox, that is unplugged. It is a bright green color, two blade-shaped pins, with one pin perpendicular to the other (so they kinda form a 'T').

I'm asking if anyone knows which connector this is (I have the service manual), and if there's any chance (at all) that it's supposed to be unplugged (before I plug it back in).

I had my pax-side airbag inflator replaced last year, so maybe it's from that ? But nothing seems to be wrong with my car (except the P0037, which problem is way more recent than the airbag replacement).

Thanks !
 

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2006 2.5i and 2002 3.0 wagons.
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Sounds like the diagnostics connector, do not plug in. Makes car go crazy pants.
 
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In the FSM the two green connectors are called "Test Mode Connector". They are part of the Bulkhead (in cabin) Harness, and are identified as connectors B75 and B76.

As has been mentioned, they are not normally connected.
 

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DTC P0037 HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 2)

Probably a failed Rear O2 Sensor heater, although could be a wiring issue elsewhere. If you haven't already, check the continuity of the rear O2 Sensor heater. See the P0037 diagnostic troubleshooting pages in the FSM. [Engine H4SO/Diagnostic Procedures with DTC Code/P0037]
 

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Discussion Starter #6
DTC P0037 HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 2)

Probably a failed Rear O2 Sensor heater, although could be a wiring issue elsewhere. If you haven't already, check the continuity of the rear O2 Sensor heater. See the P0037 diagnostic troubleshooting pages in the FSM.
Thanks much. I unplugged the connector and probed the pins in the connector going to the sensor; I got 100Kohms between pins 1 and 2 - I think I identified the pin #s correctly, but at any rate, I had infinite impedance between all other pins. So it seems the sensor heater is broken.

Tempted to just replace the sensor (I gather it's better to spring for the $100+ OEM one), but stopped short a little by the thread where the guy saw the same thing and replacing it didn't work, though it sounds like he had deeper problems ("please help with P0037 code").

Thanks much.
 

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. . . So it seems the sensor heater is broken.

Tempted to just replace the sensor (I gather it's better to spring for the $100+ OEM one), but stopped short a little by the thread where the guy saw the same thing and replacing it didn't work, though it sounds like he had deeper problems ("please help with P0037 code"). . .
Yes, in that thread the problem appears to have been a faulty engine wiring harness.

The FSM Diagnostic pages for P0037 include testing for open and grounded wiring, which is what @Nemick ultimately did to isolate the problem. (http://www.subaruoutback.org/forums/109-gen-3-2005-2009/369794-please-help-p0037-code.html ) (It's possible that the grounded wire, which might have been intermittent, could have caused the sensor heater to fail. When the code appeared, the sensor heater was open circuit, leading to replacement of the sensor, but the grounded wire was still there, causing the code to reappear.)

At this stage, if you have a multimeter, check for resistance at pin 1 (to ground) of the engine harness connector (power off). In the linked thread, it measured around 1 Ohm, indicating a short to ground. (The thread has photos identifying the engine harness connector pins.) If the resistance is low, disconnect the connectors at the ECM and check again. If the resistance remains low, then the problem is likely in the harness. (But I'm getting too far ahead -- if the resistance at pin 1 is good, then no need to go further.)
 

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Discussion Starter #8
Yes, in that thread the problem appears to have been a faulty engine wiring harness.
Yeah, that poor guy. He was lucky to find this forum (i.e. you).

... check for resistance at pin 1 (to ground) of the engine harness connector (power off).
I'm getting 2.2Kohms from pin 1 (on the engine harness connector) to GND; so looks like I'm good there. I also tried jiggling the wires from the connector to the sensor, in case some bad wiring there (would technically require replacing the sensor, but wires could be repaired); but it didn't change the nearly-infinite resistance of the sensor heater.

Trying to follow the other steps in the diagnostics, I'm desperately confused. The 4-pole connector that we're probing, both sides are called B19, and B135 is back at the ECM, right ? So when it wants me to check B135/pin2 voltages in steps 2, 3, 4, and 5, I can just probe B19/pin1 instead ? But then why does it specify B135-2 in those steps, instead of B19-1 ? In step#4, I'm seeing a voltage less than 1v (bad ECM) with the engine off, so apparently the running engine carries over from step #2 ? In step#2, how do I probe before I've disconnected B19 ? Lotta questions, sorry.

Anyhow, I'm fairly convinced it's the sensor. Is correct part# 22690AA81A ? All the parts websites just call it "oxygen sensor" (without "rear").

Thanks so much.
 

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That looks like the right part number. It's described as Oxygen sensor, whereas the front one is the Air-Fuel sensor.

(Won't have a chance to go through the details of the troubleshooting tree. Will try to later, but in the meantime the sensor probably has to be replaced in any case.)
 

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So when it wants me to check B135/pin2 voltages in steps 2, 3, 4, and 5, I can just probe B19/pin1 instead ? But then why does it specify B135-2 in those steps, instead of B19-1 ?
Probably because if there's a break in the wiring from B135-2 to B19-1, the measurements at B19-1 will not be reliable, and that would mean going to the ECM to confirm.

But fundamentally B19-1 could be used. For steps 2 and 3, the O2 sensor is connected to B19, so the measurement would have to be made by back-probing at pin 1. For step 4, the sensor is disconnected from B19, and the measurement is made at pin 1 of the engine harness connector. Step 5 is at B19.

In step#2, how do I probe before I've disconnected B19 ?
As noted, the sensor is plugged into harness connector B19 and has to be back-probed to make the measurements of steps 2 and 3.

In step#4, I'm seeing a voltage less than 1v (bad ECM) with the engine off, so apparently the running engine carries over from step #2 ?
I'm not sure of that test. Seems odd.

However, with sensor connected to B19, the engine off, but the key at On, there will be 12 V at pin 2 of B19 (coming from the oxygen sensor relay). But because the heater is open circuit in your case, there will be little or no voltage at pin 1. When the heater is good, the 12 V from pin 2 goes through to pin 1. When the engine is started, the voltage at pin 1 should drop low, but slowly increase.

Look at this post where I made some measurements at pin 1 (back-probed) with key on engine off, and then after the engine was started.
 

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Discussion Starter #11
By "back probing" do you mean checking pins while the connector is still plugged in, presumably by sticking a very thin probe into the back of the connector ?

Can you confirm that various instructions in the debug procedure, having to do with the ignition key, engine status, and connector plugged/unplugged-ness, carry over from one step of the procedure to the next ?

Thanks as always !
 

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By "back probing" do you mean checking pins while the connector is still plugged in, presumably by sticking a very thin probe into the back of the connector ?
Yes. There's probes made specially for this (e.g. https://www.amazon.com/Lisle-64750-11-Piece-Probe-Alligator/dp/B00AUIAVW4/ref=sr_1_6?ie=UTF8&qid=1512474725&sr=8-6&keywords=back+probe+pins ) or you can jury rig something on your own. I believe there's also videos on YouTube related to back-probing in automotive applications.

Be sure that the back probe and meter wires to it cannot short to anything or get caught. The O2 sensor connector is near the right side radiator fan, so be sure to securely route the test lead(s) away from the fan area.

Can you confirm that various instructions in the debug procedure, having to do with the ignition key, engine status, and connector plugged/unplugged-ness, carry over from one step of the procedure to the next ?
Confidently, no.

Nevertheless, My reading of it is that steps 2 and 3 are done at essentially the same time with the engine running. However, I find the spec, i.e., the voltage should be less than 1 V, odd given that the system works on a duty cycle basis (and based on my own measurements in the linked post).

Similarly, step 4, as noted earlier, is problematic. For sure, the engine and ignition switch would be off to disconnect the O2 sensor from the engine harness. Then, to measure voltage the ignition would have to be back on, and possibly the engine, but it's not stated. Again, the spec, i.e., if the voltage is less than 1 V there's a problem with the ECM, is also odd. As noted earlier, if the sensor heater is open circuit, or the sensor is disconnected, then the 12 V from the sensor relay can't get to pin 1, so there wouldn't be any significant voltage at pin 1/B136-4. Consequently, I can't see how the "bad ECM" conclusion of step 4 is arrived at.

Step 5 (checking for 12 V at pin 2) I believe is self-explanatory, as is step 6 (sensor heater resistance).

Interestingly, the FSM for 2007 does not use those same measurements at all. Rather, it focuses on verifying the availability of the 12 V from the relay at pin 2 of sensor harness connector, on wire continuity between the sensor connector (pin 1)and the ECM, on ECM grounds, and on the resistance of the sensor heater itself.
 

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Discussion Starter #13
Hmm, ok, it's all a bit odd. Yes, I recall from that other long post that the ECM uses a PWM signal to regulate the heater; another link I found shows B19/pin1 going into the ECM and thru a resistor and then an NPN transistor to ground (which would be a way of applying the PWM).

Anyhow, I'm pretty convinced my sensor is the problem, and that I won't fry a new one if it's not (as least not fry it in the same way as that other guy did :) ).

That's interesting about the '07 FSM; maybe they realized the one we're looking at (mine is '05, actually, even though my car is '06) didn't exactly make sense and revised it. When you say it focuses on ECM grounds, does that include the other guy's issue of B19/pin1 being shorted to GND ?
 

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. . . When you say it focuses on ECM grounds, does that include the other guy's issue of B19/pin1 being shorted to GND ?
No. The ECM has a number of grounds for it's circuits. They each have a pin at the ECM connectors, and wires that go to a ground point nearby on the car body. It's those grounds that are checked, because (I presume) if a particular ground is broken, then the related circuit would not work properly.

another link I found shows B19/pin1 going into the ECM and thru a resistor and then an NPN transistor to ground (which would be a way of applying the PWM).
Interesting. Do you have the link? (A grounded emitter NPN type configuration would explain why, when the sensor is disconnected, there isn't any voltage at B19-1 or the corresponding pin at the ECM, contrary to what step 4 implies.)

(mine is '05, actually, even though my car is '06)
Although both are 3rd Gen (Outback), there are some significant differences and probably a lot of smaller changes. Bad enough that the FSM for a particular model year can go through several revisions from first issue to final (the 2007 FSM ended up with something like 35 versions between late 2006 and 2013), it's probably not a good idea to depend on the FSM for a different year if it's not apparent where there are differences. (Few if any of the pages identify the year or version, consequently, unless one knows exactly which version it is, there's always a possibility that the one being used was an early one that had been subsequently replaced.)
 

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Discussion Starter #15
Interesting. Do you have the link? (A grounded emitter NPN type configuration would explain why, when the sensor is disconnected, there isn't any voltage at B19-1 or the corresponding pin at the ECM, contrary to what step 4 implies.)
https://www.engine-codes.com/p0037_subaru.html

BTW, in step 4, you're damned if you do and damned if you don't. In other words, once you're at step 4, you either have a bad ECM or a problem in the wiring ("yes" or "no" to "voltage less than 1v"). Same for step 3. So if ECM and wiring are good, steps 3 and 4 should be skipped by a "yes" (B19/pin1 less than 1v with engine running) in step 2. Which makes sense, if the engine and sensor are cold, the BJT would be pulling B19/pin1 low most of the time. But with the engine cold, it should not be shorted to GND (BJT is off), as was the case with the guy who fried his new sensor; the FSM doesn't seem to address that fault.
 

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Discussion Starter #16
My reading of it is that steps 2 and 3 are done at essentially the same time with the engine running. However, I find the spec, i.e., the voltage should be less than 1 V, odd given that the system works on a duty cycle basis (and based on my own measurements in the linked post).
Maybe we already agreed on this, but it makes sense to me. Since the heater is energized when B135-2 (connected to B19-1) is pulled low by the BJT in the ECM, then when the engine is cold, the PWM signal is going to have a duty cycle where that pin is pulled low most of the time (to bring the heater up to temp). So just reading with a meter, the voltage would look very low.

Since my heater is open-circuit, I'm not sure it's at all predictable what the voltage at B19-1 would be with the ignition on. I haven't taken any measurements with the engine running.

Ah well. I've purchased a new sensor and will report back after I've installed it.

Another question: I assume this problem will stored in the OBD "history". Will that clear eventually with repeated driving cycles ? If not, am I likely to have a problem at my next safety/emissions inspection if I don't get my history cleared ?
 

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Since the heater is energized when B135-2 (connected to B19-1) is pulled low by the BJT in the ECM, then when the engine is cold, the PWM signal is going to have a duty cycle where that pin is pulled low most of the time (to bring the heater up to temp). So just reading with a meter, the voltage would look very low.
That's correct, but only initially. As I noted in the other thread:

"The engine was cold. As soon as the engine was started, pin 2 jumped up to ~14.3 V (alternator running), and pin 1 dropped down to ~1 V. After about 20 seconds, the voltage on pin 1 moved up and oscillated between ~10.5 and ~5 V, the oscillations being caused by the ECM switching the heater on and off (low voltage at pin 1 = heater on)."

I assume this problem will stored in the OBD "history". Will that clear eventually with repeated driving cycles ? If not, am I likely to have a problem at my next safety/emissions inspection if I don't get my history cleared ?
If the fault is corrected by replacing the sensor, the CEL will go off automatically after three consecutive drive cycles. The code, however, will be retained in memory for up to 40 drive cycles.

Not sure about the emissions inspection. Depends on what the state/province requires. However, the difference between an active code with the CEL on, and old codes that are just stored should be recognized. As long as the CEL is off, the stored codes should not matter, but then again, it would be up to the relevant regulations.

If codes are cleared using a code reader or by disconnecting the battery (these clear the active code, as well as older stored codes, and ECM learned adjustments), it will also clear other emission-related "monitors" (tests the ECM does regularly "in the background"). These can take up to 100 miles of varied driving cycles to restore. Some emission testing regulations check to see if the relevant monitors are "ready", and if they are not, the test will not proceed. Consequently, if the memory is cleared, it might require more time before the emission test can be passed. If your emission test isn't for some time, and the CEL being on for a few drives, just leave it to follow its course.
 

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Discussion Starter #18
That's correct, but only initially. As I noted in the other thread:

"The engine was cold. As soon as the engine was started, pin 2 jumped up to ~14.3 V (alternator running), and pin 1 dropped down to ~1 V. After about 20 seconds, the voltage on pin 1 moved up and oscillated between ~10.5 and ~5 V, the oscillations being caused by the ECM switching the heater on and off (low voltage at pin 1 = heater on)."
Seems reasonable to me. I'd expect the thing to heat up pretty quickly, so seems reasonable that even after 20sec the duty cycle would change enough that <1v would no longer be seen.

The thing I wonder, what does the ECM use to decide what the temperature is and therefore what the duty cycle should be ?


If the fault is corrected by replacing the sensor, the CEL will go off automatically after three consecutive drive cycles. The code, however, will be retained in memory for up to 40 drive cycles.

If your emission test isn't for some time, and the CEL being on for a few drives, just leave it to follow its course.
Thanks for explaining. My emissions test isn't until June 2018, so I should be cool with 40 drive cycles, even as little as I drive these days (retired and "putter around the house" a lot). But your second quoted comment suggests that the longer I've seen the fault, the longer it'll stay in memory; do I misunderstand ?
 

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. . . The thing I wonder, what does the ECM use to decide what the temperature is and therefore what the duty cycle should be ?
Not sure about rear O2 sensor, but in the case of the front A/F Sensor, the ECM monitors the "resistance" of the A/F sensor element itself. The resistance of the sensor is a function of its temperature -- the higher the temperature, the lower the resistance. The sensor has to be hot to function properly.

In my 07 (and probably your 06), the A/F Sensor Resistance indication (read from the ECM) starts out cold at around 255 Ohms (which I believe is the max the ECM will register -- the actual resistance is probably higher) and it will drop to around 31 Ohms, where it normally remains thereafter within a few Ohms. (See attached chart.)

Interestingly, my measurements of the rear O2 sensor cited earlier showed the voltage being low for the first 20 seconds or so, and then increasing to a changing mid-range. I would imagine that the rear O2 heater control and the front A/F sensor heater control function in a similar fashion,
i.e., the ECM monitors the sensor element resistance, and controls the PWM accordingly. (Although there's no similar ECM readout of rear O2 sensor resistance.)

Incidentally, the A/F sensor resistance chart, and my earler measurements of the rear O2 control voltage, were probably made with the car initially at around room temperature, i.e., 60 -70 F. But in winter, when it could be near or below freezing, the time it takes for the A/F Sensor resistance to start dropping is much longer. In some cases it has been somewhat over a minute.

Thanks for explaining. My emissions test isn't until June 2018, so I should be cool with 40 drive cycles, even as little as I drive these days (retired and "putter around the house" a lot). But your second quoted comment suggests that the longer I've seen the fault, the longer it'll stay in memory; do I misunderstand ?
I left out a few words in: "If your emission test isn't for some time, and the CEL being on for a few drives, just leave it to follow its course." It should have read: "If your emission test isn't for some time, and the CEL being on for a few drive cycles, i.e., 3, isn't a bother to you (e.g., cruise control is not available when the CEL is on), just leave it to follow its course for self-cancelling."

As far as the memory retention, no, that's independent of how long the fault was present. If the fault is repaired, the CEL will be turned off after 3 consecutive drive cycles with no faults detected, and the code will be cleared from the longer-term memory after 40 drive cycles.
 

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