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Discussion Starter #1
Hello guys,

How are you today?

My car is a 2003 Outback 2.5L N/A. I'm troubleshooting a P0036 code, Rear O2 sensor low voltage on heater control circuit.

I have a brand new O2 sensor and new wiring from the O2 clip to the engine harness. This P0036 code was present before I replaced the sensor; it was present even when I had no O2 sensor installed. When I replaced the O2 sensor and reconnected the battery, the code returned within seconds. I cleared the code multiple times and every time it returned shortly after.

I have checked continuity of the O2 sensor and sensor wiring to the engine harness, and I have checked voltage of the heater circuit at the engine clip. I am very dumb with electronics, but it seems that everything checks out. With the car on, the circuit reads ~13 volts at the engine clip.

With my FreeSSM software, I ran a test of the rear O2 heater circuit voltage with the car running and the test said the circuit was at a totally constant 0.2 Volts.

I'm pretty much at a loss now. This is the only electrical issue that I have with this car, as far as I can tell. Am I looking at a problem with my ECU? I don't know enough about how this stuff works.

Thanks guys.
 

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I don't have a listing for P0036 in the H4 version of the 2003 FSM I have. Are you sure that's the code?

Also, I'm not certain but I think the FreeSSM "Rear O2 Sensor" readout of a constant 0.2 V is low. I believe it should vary as the engine is revved up and down, typically between 0.2 and 0.8 V. (I'm open to correction on this.)
 

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Yeah I don't list the P0036 either.

If you're getting a heater circuit low code, there is probably an open in the circuit somewhere.
As Saint J VT pointed out:
-check for good grounds to the ECM
-check the heater circuit inside the O2 sensor isn't open (new parts can be bad out of the box)
-check for 12v at the + side of the heater (sounds like you already did this)

If you have tools capable of measuring amperage, the heater circuit should pull ~1.1A during operation.
 

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Discussion Starter #5
Sorry guys! P0037 is the correct code. My mistake.

Is the heater circuit supposed to fluctuate in voltage? I knew the signal is supposed to but I figured the heater circuit would only change as the exhaust warmed up.

I've already checked the O2 sensor heater circuit for continuity and it's good. The wiring from the engine connector to the O2 sensor is also good. I checked for 12V at the engine connector's (+) pin and I got it.

I'm not really sure what ECM grounds to check, I'm a noob at this, so I'm not sure where to start. I mean, the heater circuit ground (-) pin at the engine connector works fine.
 

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Discussion Starter #6
I've researched a bit more about how the ECU grounds and I've decided that my next course of action should be to check the ground pin on the ECU, connector B134, terminal 34 is the ground for the heater circuit, according to THIS. I guess I could check to make sure that it is grounded properly.

Perhaps I'm going about this all wrong...
 

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The link isn't working.

Is the heater circuit supposed to fluctuate in voltage? I knew the signal is supposed to but I figured the heater circuit would only change as the exhaust warmed up.
No, the heater voltage is around 12 V. (However, earlier you referred to using FreeSSM to check the heater Voltage. I don't think there is a rear sensor heater Voltage readout in FreeSSM. As far as I know, the only rear O2 sensor parameter that can be read by FreeSSM is the sensor signal Voltage. But I don't have a 2003, so I might be wrong.)

The heater is supplied with 12 V at pin 2 of the 4 pin O2 sensor connector whenever the ignition key is at ON. (This is through the "main relay". The same 12 V goes to the front A/F sensor.)

The sensor heater current is controlled through the ECM, which switches the ground side. This, I believe, is at pin 3 of the 4-pin sensor connector.

The other two pins, 1 and 4, are across the sensor itself, and that should measure about 30 Ohms.
 

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Yes, the link seems to be working now.

That table -- is it from a 2003 service manual? It looks like the format used earlier, and things do change. The connector and pin numbers are different from the FSM that I have for 2003. I've attached the P0037 troubleshooting section. Maybe it might help.

The 0.2 Amps for the heater current appears to be correct.
 

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cardoc is far more familiar with the "ways" of air/fuel and oxygen sensors. I'm trying to get a handle on what the P0037 really means, and will put out my idea, but if cardoc, or anyone else thinks I'm wrong, please do comment.

There's a schematic of the rear sensor wiring in the first attachment above. In regard to the heater, 12 V (or thereabouts) comes from the Main Relay to pin 2 of the 4-pin sensor connector. Current in this path goes through the heater to sensor terminal 1, and from there to ECM C4 (B136, pin 4). The ECM controls the heater on/off by connecting C4 to ground to turn it on, and opening the circuit (in the ECM) to turn it off.

The only place in this circuit that the ECM can actually test for heater circuit function is inside the ECM itself. It can do this two ways: It can measure the actual current through pin C4 to ground when the internal switch is open (heater off) and closed (heater on). In this case, the current will be "high" when the heater is on, and "low" when it's supposed to be off.

Another way to verify the system is to measure the Voltage at pin C4. In this case, when the ECM grounds the pin to turn the heater on, the Voltage should be low, and when it opens the ground to turn the heater off, the Voltage will be high.

My understanding of P0037 is that it is indicating an abnormal "low"; that is, when the ECM internal switch grounds pin C4 the voltage is low as it should be, but when it opens the ground to turn the heater off, the voltage at C4 remains low, when it should go high.

The voltage at C4 can remain low if there's an internal problem in the ECM where, although the ECM commands an open circuit, the internal switch remains closed. This is theory, and would be very unlikely. The ECM is quite robust and failures are rare.

However, a "low" at C4 could occur if the sensor heater is open, or, the power circuit from the relay, to the sensor and from the sensor to the ECM at C4, is open.

But what if the ECM is actually sensing the current through the C4 to ground connection, rather than the voltage at C4. A "low" would mean little or no current when there should be current. This can be caused by defective internal switch (again, unlikely) but also by an open circuit somewhere between the relay and ECM C4.

This suggests that the most likely cause would be an open in the relay-heater-C4 circuit.

Troubleshooting should include checking for the 12 V (it might be closer to 10V according to the attachment) at pin 2 of the 4-pin sensor connector, and I believe that with the key at On but the engine not started, the 12 V should also appear at pin 1, and at C4 of ECM connector B136. (I believe the ECM will normally ground pin C4, causing it to go "low" only when the engine starts. But I might be wrong on this.)

Also, as noted in the troubleshooting chart, the heater resistance can be checked (~30 Ohms) at pin 2 to ground of the sensor connector.

I should note that for in my 2007, the sensor heater isn't turned On in a permanent way. Instead, the ECM turns it on and off at a rate of about 4 times per second. Consequently, any voltage measurements will be skewed by the way the measuring meter deals with rapidly changing signal. A 12 V that switches on and off at this rate might be read out as 6 V or 8 V, depending on how the meter averages. If this applies to the 2003 as well, then a normal reading would be in that range, a "high" would be closer to 12 V, and a "low" would be closer to zero.

Finally, a short to ground between sensor pin 1 and ECM pin C4 can also cause a "low" at C4, so this section should be checked for any grounding. (Again, it's rare that a problem develops in the wiring harnesses.)

Incidentally, the CEL will come on when the fault is detected in two consecutive driving cycles. It will also remain on until the completion of 3 consecutive driving cycles without the same (or another) fault being detected, after which it will go off automatically. The code will remain in memory for 40 driving cycles (even after the CEL goes out). All, of course as long as the code isn't manually deleted from memory.

In light of this, the immediate return of the code (post #1) is puzzling, and leads me to wonder if my explanation is correct. I think we need to understand how the OBD works, and sets codes, to be better able to narrow down the problem. As cardoc has demonstrated, sometime a code is caused by something totally distinct from the identified system. (The example was a P0420, which is related to the catalytic converter and it's sensors, which was found to be caused by a defective engine coolant thermostat.) I wonder if this could be the case here.
 

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Discussion Starter #12
Oh man, plain OM you are a truly great resource. I have found the problem and it makes me feel stupid and sorry to make you write so much.

The replacement harness was not good. I shouldn't have said it was a new harness; it was a new-to-me harness from a part-out. It turns out that for some inexplicable reason, three wires were switched on the harness connector that clips into the O2 sensor itself and I didn't notice it. The only pin that was in the right place was the Yellow/Red 12V+ wire. I switched the pins around to where they were supposed to be and it works great. I guess I didn't notice it while I was testing the wires because I wasn't paying attention to which wires were going where. The way I figured it out was the thing I should have done off the bat. I plugged the O2 sensor directly into the engine harness and found that there was no code. That isolated the problem to the harness between the engine and the sensor. I realized that the wire colors did not match up on the sensor end, fixed them, and passed emissions less than an hour ago.

God, I feel dumb. But I have learned today. I have learned to always eliminate the basic things before tearing into more complex, less likely things. I'm sorry to make you write all that, but I thank you for your help! If it's any consolation, people reading this in the future who actually have a problem more extensive than mine will find your analysis very helpful.

Thanks a lot.
 

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Great news, and congrats on the troubleshooting.

No problem about the post -- it was, and is, "thinking on paper" which is good to do and, as you note, maybe help others in future, if it's correct!

It also goes to show that at our end we tend to accept the printed word without question; "new" is new, right? But as you noted, new can be relative, and needs definition.

It does perhaps confirm the theory . . . , well sort of. The 12 V was getting to the heater (presumably at sensor connector pin 2) but perhaps not to C4 at the ECM, consequently, the continuous "low".

How did you know which wire colors in the sensor wire harness should go to which connector pins, and that they were mixed up?
 

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Discussion Starter #14
There were a few things that hinted to me that the wiring was screwed up. It was a novice's progression, for sure. First, I didn't pay close enough attention to the pin numbering, so when I found two pins that represented a circuit by my ohmmeter, I just assumed that the heater circuit was good, without realizing that the pins I was reading were not matched up to the heater circuit wiring on the main harness clip. Eventually, I noticed that the side of the harness with the O2 clip had the wires leading to pins in a pattern that did not correspond with the clip at the engine side of the harness.

I did some research, and found the correct orientation of wires (this orientation WAS already correct at the engine side of the harness). I used the O2 sensor's layout to confirm this orientation.



Also, the black "goo" that the factory uses to keep the wires together was torn at the sensor end of the harness so that the individual wires were free. So, it was apparent that the wires of the sensor side had been altered at some point. I rearranged the pins in the sensor side of the harness to match the layout of the engine side of the harness and all is well. I hope that makes sense, I am trying to explain it as well as I can.
 

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Good sleuthing for that info on the wire colors and positions.

Ever thought of getting a FSM for your 2003? With a low price of ~$35 (but lots of individual files to download) from the SOA site (Subaru Technical Information System - Welcome), it's might be well worth it if you're going to do more of your own work on the car. All the wiring diagrams (with colors!), connector layouts, input/output tables, as well as mechanical work, maintenance etc can be included in what will be the latest, up-to-date version from the mfr. Just a thought . . .

Enjoy!
 

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Discussion Starter #17
Haha I actually have an FSM that I downloaded from Subaru. However, I have only printed out sections of it that I've needed (mostly mechanical engine information), and the rest of it is on a desktop that is not currently with me. The FSM would have made it a good deal easier, I'm sure. There's no way I could have done the engine work that I've done without it.

Thanks again!
 
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