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Old 04-28-2019, 06:15 PM   #1
Lottadro
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Default 11v at pin 4,5 connecter b137

So Ive been going through fsm for front o2 issue. I get to step 5 in the p0031 diagnostic process and Im reading 11vs at pins 4,5, back probing. Im think ecu should be regulating to less than 1v, correct? Is ecu shot then?

Any insight appreciated.

UPDATE

I found the issue to be injectors 1 or 3. The were dumping fuel in, the front o2 saw this and was pulling fuel, negative trim. However, this caused the rest of the engine to be lean, a false rich condition. So the trims were negative even when the AFR was swinging lean.

Special thanks to Cougar4 and the guys over at ScannerDanner. I wouldn't have been able to come to the conclusion without you.

Thank you.
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Last edited by Lottadro; 05-08-2019 at 09:12 PM.
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Old 04-28-2019, 10:13 PM   #2
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Cougar4? Mulder?
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Old 04-29-2019, 12:16 PM   #3
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What is the function of each pin? I assume the pins you are talking about are on the ECU and if so they should have a letter with the number also.

Code P0031 is a faulty heater circuit code for the front O2 sensor. There should be 12 volts on the heater circuit at least while the sensor is getting up to temperature. I would suspect a problem with the wiring to the sensor or the sensor itself before blaming the ECU for a problem. The wiring to the heater and the heater element can be ohm'd out to see where the problem is.

Last edited by Cougar4; 04-29-2019 at 12:22 PM.
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Old 04-29-2019, 01:46 PM   #4
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Cougar4, They are on connector B137, so I believe they are D4, D5. These connect directly to pin 1 on the o2 sensor harness. On the FSM, it states it should be at 1 or less than 1V. I tested resistance on all the sensor wires between sensor and ECM, all were fine.

o2Harness ECM
1 D4, D5 White
2 Main Relay Yellow/Red
3 D29 Black
4 D19-18 Shielded

Last edited by Lottadro; 04-29-2019 at 01:55 PM.
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Old 04-29-2019, 03:49 PM   #5
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D4, D5 would be signal wire.
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Old 04-29-2019, 04:12 PM   #6
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I am not exactly diagnosing a p0031. I have found that with the front o2 sensor plugged in I have horribly rough idle, misfires on all cylinders (p0301-4), p0037 (even though its defeated in tune), and my AFRs on AP range from 12-19 while my UEGO reads 13- to max lean. Unplugging the front o2, forcing open loop, completely negates all of these issues. The only codes then are p0031 (unplugged) and p0037.

Following this logic, I began to test the wiring for the o2 sensor, which is when I found the signal wire on D4 and D5 to be getting 11Vs at idle. Now, this is my main question: is the ECU supposed to regulate the voltage to the signal wire? If so, and the FSM calls for 1V or less at idle, then this would lead me to believe that the ECU is done.

Thoughts?
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Old 04-30-2019, 02:41 AM   #7
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Sounds like your sensor is roached.

And what I mean by that is the FSM says pin 1 is "signal" but what they mean is it's the ground for the heater. The ECU monitors the voltage on the ground to determine if it's functioning properly, hence "signal".

If you have 12.4*v on pin 2, and 11v on pin 1, you've only got a 1.4* volt drop through the major load on the circuit.




*Assuming 12.4 for argument's sake. YMMV.
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Old 04-30-2019, 10:08 AM   #8
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Cosmo, I have tested with three different sensors, one brand new (denso). All three times I am getting 11v at pins D4 and D5. I back probed at the sensor connector and still get 11v. I checked resistance between D4, D5 and pin 1on o2 sensor connector and it is fine.

Unless I am not understanding what you are saying, I dont see how D4 and D5 should read 11Vs at idle when FSM is calling for less than 1v. I should be seeing .1-.9v fluctuating with the sensor reading the mixture, correct? I have tested resistance between pin 1 and 2 on the sensors and all are within spec of under 10 ohms at 1.2-4ohms.

What also stumps me is why p0037 is popping up when it has been defeated, in either case of the front being plugged in or unplugged.

If it is simply that the original Denso sensor is shot, and the new Denso is shot right out of the box, and my original 3rd Subaru sensor is also shot, I guess I'll just have to get a fourth sensor and wonder when the quality of production fell off the cliff. Thats two Densos, one OEM Subaru sensor. Though, as I stated above the resistance on all 3 sensors' pins 1 and 2 are within spec.
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Old 04-30-2019, 10:50 AM   #9
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Ok, with all that stated above, I started looking at the p0032 code diagnostics, which I think is actually a better lead to follow, after my findings at D4,D5.
Step 1: is voltage at D4, D5 higher than 8Vs ----------------> in my case, yes 11v - Go to step 3
Step 3: Does the voltage change by shaking harness ------> not really, stays at 11v - Repair battery short circuit in harness between ECM and front o2.

Step 3 confuses me. Which wire would I be fixing? Like I said, resistance is fine on all wires between the connectors, there was no OL. Am I looking for an issue on pin 2 from the o2 connector, Yellow/red wire to main relay? Am I looking for an issue between pin 1 from o2 connector, white to D4, D5? How is there an issue with the wiring if the resistance is fine? I am asking honestly here, I really don't know.
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Old 04-30-2019, 12:04 PM   #10
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The way I understand the circuit for the O2 sensor is pins 2 and 1 are for the heater of the sensor. Pins 3 and 4 are for the sensor signal to the ECU. Pin 1 supplies a return to the battery for the heater circuit which is controlled by the ECU. Pin 2 supplies the power. There should be around 12 volts across pins 1 and 2 while the heater is ON. If there is 12 volts on pin 1, using ground as your reference, then the heater is OFF. There will be very little voltage on that pin only when the heater is ON since it will be near ground reference at that time. After the sensor is heated up to normal operating temperature the ECU turns the heater circuit OFF. Here is a good explanation of the operation.

P0031 Subaru Description
Oxygen Sensors (O2S) or Heated Oxygen Sensors (HO2S) need to reach a minimum operating temperature of 750 degrees F to produce an accurate voltage signal. The faster the heated oxygen sensor reaches that temperature the faster the sensor will start sending an accurate signal to the Engine Control Module (ECM).
In order to achieve the require temperature, a heater element is included inside the heated oxygen sensor. The ECM controls the heated oxygen sensor heater element based on signals from the engine coolant temperature and engine load. The ECM controls the heater element circuit by allowing current flow to ground.

The ECM monitors the voltage signal received through the heater element circuit and determines the state of the circuit by comparing the voltage detected with the factory specifications.


It seems you have already tried installing a new sensor and it didn't make any change to the issue. If you haven't already checked the continuity of pins 3 and 4 to make sure those wires are okay back to the ECU then you need to do that. Those wires are also shielded to prevent noise getting onto the leads. The trouble you described kind of hints that there may be a problem there. Make sure the shield wire is connected to the proper ECU pin. Along with making sure each wire has good connection at each end, you also should make sure that the sensor wires are not bridging together somehow. They need to be isolated from each other, along with any power or ground connection. Pin 2 will have power on it when the ignition is ON but the other 3 leads should be totally isolated when both ends of the connections are disconnected from thngs.

Last edited by Cougar4; 04-30-2019 at 12:16 PM.
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Old 04-30-2019, 04:22 PM   #11
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Cougar, I checked continuity of pin 3 going to D 29 (black wire) tested good. Pin 4 going to D 19 (white wire) also tested good. The shielded wire is at ECU connector pin D 18 (yellow), I tested to both pin 3 and 4 and got OL. Going off the diagram, I am testing at the right pins, however, I am not 100% certain that the shielded wire at D 18 can be tested for continuity, unless it is connected mid-harness. Maybe, even after looking at the shielded wire explanation in the FSM, I am not certain how its actually constructed. If it could be tested for continuity, and it is in fact shorted somewhere in the harness, that would be a great step in the right direction. Any instructions you could pass my way would be greatly appreciated. Thanks for your help thus far.

But if we are checking on the p0032 with key ON, voltage should not be above 8v for D4 D5. p0031 reads as though it should be less than 1v I am not sure what to think here. I get what you are saying about the circuit of the heater. However, the FSM seems to be indicating from its steps that there is a battery short circuit between ECM and front oxygen connector.

My head feels like a mess trying to keep this sorted.

PS: You were right, new ECU didn't fix.

Last edited by Lottadro; 04-30-2019 at 04:58 PM.
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Old 04-30-2019, 10:00 PM   #12
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When the engine is cold, the voltage on pins D4 and D5 of the ECU should be close to zero volts while things are connected up normally. That means ground is connected to those pins (inside the ECU) which will allows current to flow through the heater element, which is tied to power on pin 2 of the sensor. I assume you have 12 volts on pin 2 (yel/red) of the sensor when the ignition is ON.

To verify that there is no connection between the power wire to the sensor and the return wire to the ECU remove the connection at the sensor with the ignition OFF. Check the resistance between pin 1 and pin 2 of the sensor connector. You should see a high resistance between those pins. Meaning there is no connection between the wires. If the resistance is low then there is a bridge in the wiring somewhere and that needs to be fixed.

Last edited by Cougar4; 04-30-2019 at 10:26 PM.
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Old 04-30-2019, 10:07 PM   #13
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I edited what I wrote a few times and left something out. You'll have ~battery voltage everywhere on a circuit if there's no ground, so having 11v at the D4/D5 pins could just mean that the ECU is not commanding the ground closed, so the heater isn't turning on. Or it's got really high resistance to ground.

Check continuity to ground from pins 8 and 9 on that same connector. There should be very little resistance.
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Old 04-30-2019, 10:11 PM   #14
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+1

Also check the ground rings at the rear passenger side of the intake manifold. The ecu gets its ground source (for everything it powers/controls) there, maybe it's loose and causing crazyness.
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Old 04-30-2019, 10:33 PM   #15
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Having a bad ground connection to the ECU makes the most sense. I think you guys have nailed it.
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Old 04-30-2019, 10:34 PM   #16
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First, thanks all for replying.

Cougar, disconnected sensor and harness with key OFF and tested pins 1 and 2 for resistance on harness side: OL. On the sensor pins 1 and 2 read 1.2 ohm.

Cosmo, resistance is .1-.2 on D8 and D9 to chassis ground.

2slo, I'll do so tomorrow, thanks for the suggestion.
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Old 04-30-2019, 10:42 PM   #17
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Well your resistance readings seem good to me. No bridged wire to the power. The sensor resistance seems good and the grounding to the ECU is good. So lets see what the voltage is on pins D4and 5 with the ignition ON and the engine not running. I would like you to recheck that again with everything connected normally. While the engine is cold.
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Old 04-30-2019, 11:03 PM   #18
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Cougar, just went and checked right now. Key ON with engine off and cold, pin D4 and D5, back probed, read 11.86v to chassis ground.
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Old 04-30-2019, 11:13 PM   #19
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Just like before. So for whatever reason the ECU is not grounding those pins. You already tried another ECU and that didn't help. I don't know how the ECU is connected internally to make the controlled connection to ground. I suggest you check every connection to power and ground going to the ECU and verify those connections are good. If you have a O2 sensor on bank 2 then compare the voltage on the ECU pins for that heater. I will check back tomorrow for any updates.
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Old 04-30-2019, 11:23 PM   #20
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The ecu gets its ground source from the ring terminals bolted to the engine, on the engine harness. It uses a transistor inside to connect its source ground to the O2 sensors which already have 12v positive on their other leads from ignition/key on power. You can also have the p0031 if the fuse (or 12v circuit that feeds them) that powers the O2 sensors is blown.

On some models the ground rings are on the top of the engine block rear passenger side, on others the rings are on the intake manifold on the passenger side.

Also forgot to mention to make sure the main engine harness plugs on the firewall side and engine harness side are securely connected to each other.

Last edited by 2slofouru; 04-30-2019 at 11:32 PM.
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Old 05-01-2019, 02:12 AM   #21
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Ok so your ECM ground is good. Your power is good. Your AF sensor sounds like it's fine. So why isn't the ECM commanding the heater circuit on?

Did I see you tried a different ECM? Are you loading the same tune on it? Can you put an OE tune on it and see if it will at least command the heaters on KOEO? It doesn't sound like it's anything hardware related.
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Old 05-01-2019, 01:17 PM   #22
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Cosmo, I am in the process of checking behind the head unit the prior owner installed. I want to see if anything is spliced or tapped. If that comes back negative, im going to actually uninstall the AP, returning to stock tune. Your suggestion is interesting. Perchance there is something going on there.

2slo, Cleaned ground terminals on both sides of manifold. Same reading.

Cougar4, If todays activities dont produce results, Ill move on to making a list of all power and ground connections and working through them.

Thanks.
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Old 05-01-2019, 01:23 PM   #23
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So far we haven't been able to find out what is causing the problem, even while doing some good logical testing. Just out of curiosity, I wonder what the rear O2 sensor heater circuit is doing. Can you check the voltage on the ECU pins that control the return circuit for that sensor and see if that one is working the same way as the front one is?

Last edited by Cougar4; 05-01-2019 at 01:29 PM.
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Old 05-01-2019, 01:48 PM   #24
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Cougar, Ill definitely check those when I get home. Very interesting indeed.
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Old 05-01-2019, 05:12 PM   #25
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Default 11v at pin 4,5 connecter b137

Cougar, I checked the rear o2's pin B17 (white wire) and C13 (red/white), both had very low voltage. Nothing like the front.

Cosmo, I uninstalled the AP completely, nothing changed 11.86v with KOEO.

I pulled glove box off to look at firewall entry of harness, could see nothing that looked bad visually. Head unit also looked decent, nothing really out of place. I did pull out an RCA filter that was attached to nothing.

I cant really think of anything else at this point. I guess I am going to have to make that list of power and ground pins. That'll take some time.

Last edited by Lottadro; 05-01-2019 at 07:41 PM.
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