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Hey all, I recently finished putting a 2003 wrx engine in a 98 RS using the wiring harness from a 2004 wrx, the only snag I am running into is the front O2 sensor. I am getting a P0031 code, I changed the sensor and it didn't fix the problem. I measured the voltage at the connector and got the following:

 

Small white wire-4.7v

Small black wire- 4.7v

Large white wire- 3.4v

yellow/red wire- 12v

 

I remember reading somewhere that there are two wires for the O2 sensor pinned out in the ECU differently between the years 2003 and 2004. Because I am running the 2003 ecu with the 2004 harness, this could be my problem, but I can't find it again to know which wires to change.

 

Any help is greatly appreciated!

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That is a heater circuit code which means there is either a bad connection at the sensor or at the ECU, or the sensor needs to be replaced.

 

These new sensors work a bit differently than the old ones and I'm not entirely familiar with what goes on inside them yet, but the ECU monitors how much current is being drawn by the heater in the sensor. If it finds a problem it throws a code for it.

There are several wires on the ECU for the O2 sensor. 05 FSM says pins A2 and A3 are for the heater circuit, both of those connect to pin 4 on the sensor harness connector.

I can't seem to dig up an FSM for an Imp earlier than 05 though so I'm not sure how much this helps you. I'd say if anything, 04 is probably 90% chance gonna be the same, so look on the ECU connector for wires on pins A2 and A3. Those should be tied together somewhere near the ECU.

 

I'll keep looking though to see what I can find.

(I bookmark and/or download copies of all the FSMs I can find for reference. :grin: [/nerd])

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Hey all, I recently finished putting a 2003 wrx engine in a 98 RS using the wiring harness from a 2004 wrx, the only snag I am running into is the front O2 sensor. I am getting a P0031 code, I changed the sensor and it didn't fix the problem. I measured the voltage at the connector and got the following:

 

Small white wire-4.7v

Small black wire- 4.7v

Large white wire- 3.4v

yellow/red wire- 12v

The yellow/red wire is the 12v supply to the A/F sensor heater. The "large" (heavier gauge) white one is the ECU control line for the heater. If it's working/connected correctly (pulling towards ground), you should measure under 1.0 volt there. Since it's quite a bit higher, that's what's likely causing the P0031 code to be triggered.

 

 

I remember reading somewhere that there are two wires for the O2 sensor pinned out in the ECU differently between the years 2003 and 2004. Because I am running the 2003 ecu with the 2004 harness, this could be my problem, but I can't find it again to know which wires to change.

Unless someone else knows the answer offhand, this might take a bit of detective work. I have the wiring diagram for the 2004, but not the 2003. The difficulty is that we don't know if there's a wiring difference, or just poor connections leading to a voltage drop. Even if the wiring is correct, high contact resistance could be either in the wiring from the A/F sensor to the ECU, or from the ECU to ground.

 

If you're willing to check a few things, we might be able to figure this out even without the 2003 info.

 

A2 and A3 are pins #2 & #3 of ECU connector B134 (diagram attached). Please verify that heavy gauge white wires are connected there. If so, measure the voltage at those pins relative to a good chassis ground and let us know what you get. You could also wiggle the connections at the sensor and ECU, and see if the voltage at those pins changes (which might require a helper, or long meter leads :)).

post-15889-136027645542_thumb.gif

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my 05 manual says that wires 2&3 in b134 are yellow and yellow with blue... my two heavy gauge white wires are in b137 pins 4&5. If i recall correctly all the o2 sensor wires are in b137

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my 05 manual says that wires 2&3 in b134 are yellow and yellow with blue...

That should depend on turbo versus non-turbo, etc. -- they vary.

 

 

my two heavy gauge white wires are in b137 pins 4&5. If i recall correctly all the o2 sensor wires are in b137

So far, I can't find a diagram that matches.

 

1) Does your B134 ECU connector outline match the diagram I posted?

2) What exact model did the 2004 harness come from -- was it an STi?

3) Can you post a diagram of your B137 (similar to the B134 one I posted)?

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neither my b137 nor b134 connector look like that

The harness came out of a 2004 wrx (non sti)

i can't upload attachments, maybe i need to have a certain number of posts first?

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ok, so the 2 white wires are reading 12v at the ecu when everything is hooked up and ignition on. there is no change when i rattle the connectors.

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neither my b137 nor b134 connector look like that

I didn't expect the B137 to look like it -- only the B134 might have.

 

 

The harness came out of a 2004 wrx (non sti)

i can't upload attachments, maybe i need to have a certain number of posts first?

Please download the following:

http://www.ludicrous-speed.com/automotive/impreza/fsm/Impreza-STi-2004/7._Wiring/17._Engine_Electrical_System.Pdf

 

It has diagrams for the 2004 Turbo, SOHC, and STi -- the Turbo is first. See if the ECU connectors match what you have. Let us know.

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ok, so the 2 white wires are reading 12v at the ecu when everything is hooked up and ignition on. there is no change when i rattle the connectors.

If you were connected to the heavy white wire at the ECU that's going to the A/F sensor heater, it should have read about what you got at the sensor before, about 3.4 volts.

 

Did you have the engine warmed and idling when you got the 3.4 volt reading?

 

Did you have the engine warm and idling when you got the 12 volt reading?

 

Could you check the voltage at the white wires at both the sensor and ECU, engine warm and idling, in order to verify that they're the same? Or, unplug the connector at the sensor and the one you're measuring at the ECU, and check resistance between them to make sure you're on the same wiring.

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so looking at the diagrams you provided, if you look under section 17 "engine electrical system" the very first diagram shows the o2 sensor circuit. there it is saying that the signal wires are e19 and e29 the shield is e18 and the two white wires are e4 and e5. the "e" connector on this drawing is b84.

 

i won't get to try the latest suggestions until tomorrow night as the car isn't at my house.

 

The 3.4V was when the engine was cold, ignition was on/engine off taken from the sensor's plug while the sensor was unplugged.

 

The 12V was taken when engine cold, ignition on/engine off taken from the back of the ecu plug (still plugged in) and also taken from the sensors plug while it was plugged in.

 

i did notice one more thing, the shield (pin e18) is connected to the ecu via a yellow wire, on the engine side it stops 6" before the plug and isn't connected to anything.

 

I really appreciate all the help, my brain is mush trying to figure this out. let me know if you need anything else clarified.

Edited by lushy

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so looking at the diagrams you provided, if you look under section 17 "engine electrical system" the very first diagram shows the o2 sensor circuit. there it is saying that the signal wires are e19 and e29 the shield is e18 and the two white wires are e4 and e5. the "e" connector on this drawing is b84.

Yes, I'm aware of that -- what I'd like to know is whether B134 and the other ECU connectors (including B84) shown on page WI-82 (page 3 of the PDF) match your actual ECU (since you said that the B134 diagram I previously posted didn't match).

 

Also, you had previously said that your A/F sensor wiring was to B137. Is that correct, or does it go to what's referred to as B84?

 

 

i won't get to try the latest suggestions until tomorrow night as the car isn't at my house.

 

The 3.4V was when the engine was cold, ignition was on/engine off taken from the sensor's plug while the sensor was unplugged.

 

The 12V was taken when engine cold, ignition on/engine off taken from the back of the ecu plug (still plugged in) and also taken from the sensors plug while it was plugged in.

I didn't realize that the 3.4 volt reading was taken with the sensor unplugged. I'd suggest the first thing to check is that the white wires you're measuring at the ECU are indeed connected to the A/F sensor. Next, with engine warm and idling, check the voltage on the white wire(s) again, with everything plugged in.

 

 

i did notice one more thing, the shield (pin e18) is connected to the ecu via a yellow wire, on the engine side it stops 6" before the plug and isn't connected to anything.

The shield should only be connected at the ECU end. Otherwise, it will form a ground loop and likely induce noise rather than shield it.

 

 

I really appreciate all the help, my brain is mush trying to figure this out. let me know if you need anything else clarified.

I'm glad to help, and hopefully we'll figure this out. I can understand the "mush" comment. Even though much of my time is spent doing this sort of thing, it still can prove challenging at times.

 

If anyone has a link to a 2003 WRX engine wiring diagram, it would be helpful to be able to compare it to the 2004.

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I found an FSM for 03 Forester. Not sure how close that is but it's deffinitely different than the 05 Impreza and Legacy ones I have. [...]

I've got the 03 Forester info already. Foresters are built on the Impreza chassis, so they're similar.

 

However, the WRX is a turbo (2.0L DOHC), and in that respect they're not the same as the base models, and the ECU is different as well.

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I looked around some more, and found ECU info here:

http://www.northursalia.com/modifications/misc/wiring/wiring.html

 

From the above site, '02/'03 WRX ECU pinout:

http://www.northursalia.com/modifications/misc/wiring/wrxpin.html

 

Also from that site, '04 WRX ECU pinout:

http://www.northursalia.com/modifications/misc/wiring/04_DOHC_ECU.pdf

 

For some reason best known to Subaru, they apparently changed (possibly in error) the numbering of some of the ECU connectors. It seems they are equivalent as follows:

 

02/03--04

B134--B134

B135--B135

B136--B137

B137--B84

B84---B136

 

Assuming the ECU info at NorthUrsalia is correct, and taking into account the connector numbering switch, a random check of several connections revealed no differences between the '02/'03 and the '04 ECU pinout.

 

Specifically for the A/F sensor, both signal and heater, the pin numbers are shown as the same whether for B137 on the '02/'03 info or B84 on the '04.

 

The ground distribution info for the '04 turbo ECU shows pins #8 & #9 (black/blue) as ground in the connector that the A/F sensor goes to. I'd suggest checking the voltage at those pins relative to a good chassis ground when doing the other measurements. The voltage there should be very low if there isn't excessive resistance in connectors, etc.

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wow great detective work! this afternoon after work, i am going to get the car running and up to temperature, then do a voltage and ground check. Thanks for all the help so far! i will post up later how my checks turn out!

 

coming off the main relay for the o2 system, there is a BR wire that goes to pin E10 (seen on page wi-84, section 17) what should i be reading at that pin? 12V correct?

Edited by lushy

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wow great detective work! this afternoon after work, i am going to get the car running and up to temperature, then do a voltage and ground check. Thanks for all the help so far! i will post up later how my checks turn out!

You're welcome. We'll see how this goes after you post test results.

 

 

coming off the main relay for the o2 system, there is a BR wire that goes to pin E10 (seen on page wi-84, section 17) what should i be reading at that pin? 12V correct?

Yes, that BR (black/red) wire is powered from the battery, via a slow-blow fuse (SBF-5), and should have 12 volts on it.

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I've got the 03 Forester info already. Foresters are built on the Impreza chassis, so they're similar.

 

However, the WRX is a turbo (2.0L DOHC), and in that respect they're not the same as the base models, and the ECU is different as well.

 

Of course I've been looking at all the non turbo stuff. :lol:

 

wow great detective work!

That's why he is the Master! AND Commander! :banana:

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Of course I've been looking at all the non turbo stuff. :lol:

There are just too many model variations. :)

 

 

That's why he is the Master! AND Commander! :banana:

I'd like a demotion! Otherwise, expectations are too high and I might not be able to live up to them...:dead:

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ok, engine running and up to temp

o2 signal wires 19 and 29- 4.3V

o2 heater wires (white) - 14V each

ground pins 8 and 9 - 0.04V

black and red wire i inquired about is getting 14V

 

Then i checked continuity form white wires at ecu to o2 sensor plug and its the same wire!

 

Then i checked the grounds to ground and they were .4 ohms each!

Edited by lushy

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To prep a harness for a merge any wires that "branch off" the main bundle between the sensors and ecu are snipped. Leaving you with a nice clean harness to work with. There were probably ground wires snipped to free the "engine harness" from the main dash/chassis harness.

 

The only grounds supplying the ECU and engine harness are the "ControlPower" and "MemoryPower" grounds that need to be spliced to make it run.

 

I think you need to add more grounds mid-stream to allow for a cleaner flow back to the ecu.

 

Ian

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ok, engine running and up to temp

o2 signal wires 19 and 29- 4.3V

o2 heater wires (white) - 14V each

ground pins 8 and 9 - 0.04V

black and red wire i inquired about is getting 14V

 

Then i checked continuity form white wires at ecu to o2 sensor plug and its the same wire!

 

Then i checked the grounds to ground and they were .4 ohms each!

Since the white heater wires read full battery voltage, it indicates that the ECU isn't pulling them towards ground at all. There are three possible causes:

1) The white wires could be going to the wrong ECU pins. I've now looked at many ECU pinout charts and wiring diagrams for the '03 and '04 WRX, and they all seem to agree with each other and with what you've reported. However, the ECU info from Subaru could be wrong -- I've found lots of errors in FSMs over the years, some minor, some not so minor.

2) The ECU A/F sensor heater control circuit could be defective. It's rare, but not impossible.

3) The ground wires used for the heater could be on the wrong ECU pins -- see 1).

 

 

To prep a harness for a merge any wires that "branch off" the main bundle between the sensors and ecu are snipped. Leaving you with a nice clean harness to work with. There were probably ground wires snipped to free the "engine harness" from the main dash/chassis harness. [...]

I'll admit to not being familiar with exactly what is necessary to perform the transplant that was done.

 

Lushy, did this involve a lot of snipping/splicing, as EastCoastEJ22T has mentioned?

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Okay, I spent some more time online, and found:

 

http://forums.nasioc.com/forums/showthread.php?t=883750

See the first post (ignore other two). This raises the question as to whether your heater also is coming on momentarily, which would be good to know. What I'd suggest is to leave the key off, connect your voltmeter to the white heater wires and ground, start the engine and immediately read the meter. If it's anything less than battery voltage (even for a few seconds), it means the ECU is trying to activate the heater.

 

http://www.scoobymods.com/oxygen-sensor-t13105.html

You might want to get back to these guys with the voltages you measured when the sensor is connected.

 

http://forums.nasioc.com/forums/forumdisplay.php?f=87

RS to WRX swap info

 

http://forums.nasioc.com/forums/showthread.php?t=1486051

'99 RS to '02 WRX

 

http://forums.nasioc.com/forums/showpost.php?p=25447283&postcount=2

'02-'05 WRX to '98 RS

Consider posting on this thread?

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The swap did involve alot of cutting and splicing.

 

i just checked the white wires while starting the car, and as soon as i turn the key over it goes from 12V down to 7V (for about 5 seconds) then shoots back up to 14V. so the ecu is trying to ground out the heater circuit, but doesn't seem to have a strong enough ground source maybe?

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The fact that it pulls down at all suggests that it's probably connected to the correct ECU pin(s). I wonder if the ECU is sensing something wrong and going into a fail-safe condition.

 

In your first post of the thread you said that you "changed the sensor". Did you use a new OEM one, aftermarket/universal, or used?

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