P0031 troubleshooting 2003 VDC H6

[Skip]

P0031 Oxygen (A/F) Sensor Heater Control Circuit

Low (Bank 1 Sensor 1)

 

Santa was kind enough to bring me an early Xmas gift...

CEL with the error code above.

Car is a 2003 Legacy OB VDC (with the H6)

~ 70 kmi.

 

I have searched through my .pdf versions of the FSM

but can not find the wiring for this front 02 sensor's heater circuit.

 

1) The 02 sensor could have a bad heater. (my bet)

2) The fuse for this could be bad .. but why did it blow?

3) Some charts refer to a relay for this heater

it could be bad.

What fuse block contains it and what location in the block

is this relay.

 

 

Yes I could lift the car remove the under cover find the

02 and then check the wiring for shorts.

I could then check the resistance of the heater

and if it's getting it's correct voltage.

To do this I need to know which wires to the

02 sensor are for the heater.

 

Please, if anyone has the wiring/trouble shooting section

for this heater circuit I would appreciate a link or

an email.

 

Happy Holly Daze to all.

 

 

 

Note: if you have ever seen the under hood layout of the H6,

you will have an idea why I would like to systematically

check this problem.

[OB99W]

Responses below based on info for 2001 H6.

 

P0031 Oxygen (A/F) Sensor Heater Control Circuit

Low (Bank 1 Sensor 1)

Should be RH/Front.

 

 

1) The 02 sensor could have a bad heater. (my bet)

Perhaps, or a bad connection, or a problem with the ECU (it controls the ground end of the heater).

 

 

2) The fuse for this could be bad .. but why did it blow?

3) Some charts refer to a relay for this heater

it could be bad.

Probably not fuse or relay; they feed all the O2 sensor heaters, so code for just one doesn't seem to fit.

 

 

Please, if anyone has the wiring/trouble shooting section

for this heater circuit I would appreciate a link or

an email.

I'll send something for the 2001; see your PM.

[Skip]

OB99W thank you very much for the insight and the

links via PM

 

I'll use them in my search for a solution and update as

appropriate.

 

Happy Holidays and thanks again!

[porcupine73]

Hm...I've seen this posted a couple times before for that era soob and I think in those posts it turned out to be the relay/control circuit.

[OB99W]

Hm...I've seen this posted a couple times before for that era soob and I think in those posts it turned out to be the relay/control circuit.

If you have specific info or links, especially for the 2003, I'd appreciate if you'd post them.

 

The info I have available is for the '01, so I'm not certain it directly applies to the '03. The '01 diagram indicates that the contacts on the main relay switch power to all three HO2S -- the two fronts, and the rear one. If the relay contacts are bad enough, the voltage should be low to all three heaters; P0037 and P0051 should also be thrown as well as P0031. I suppose it's possible the main relay just beginning to go bad, in conjunction with a slightly poor contact in the front right O2 sensor's heater circuit, might trigger only P0031.

 

The ECU controls the ground end of all the HO2S circuits, so voltage sensing and heater current are ultimately determined by the condition of all connections, wiring, the heaters, and the ECU itself.

[Skip]

Thanks to OB99W I have the diagrams he speaks of

and I agree with his statement.

 

Looks like the only way, aside from some corrosion,

that the P0031 code can be thrown - alone

is when the heater element in the 02 sensor goes bad.

 

 

The diagram shows the two front 02 sensors

OB99W mentions.

 

 

Heater grounds provided by the ECU at pins

4,5 and 6,7 as shown.

[Skip]

To muddy the water further

here is the diagram OB99W

linked to showing only one front 02 sensor.

Being generic OBD II codes I guess?

there is no way to indicate left and right 02 sensors.

(maybe a PO0031 and PO0032 for example only)

These are from the H6 suppliment for the 01 Legacy

[Skip]

Does anyone find fault in me starting with this

proceedure?

[OB99W]

[...]The diagram shows the two front 02 sensors

OB99W mentions.[...]

That particular diagram, since it specifically addresses P0031 (front/right low), doesn't show the wiring for the rear sensor, whose heater also connects to the same power source. Info concerning P0051 (front/left low), for the other front sensor, uses the same diagram.

 

The diagram for troubleshooting code P0037 (rear low) is different, but shows the rear sensor's heater connection to the same relay terminal as the front ones. Therefore, all three heaters are powered via the same relay contacts (at least on the '01 ).

[OB99W]

To muddy the water further

here is the diagram OB99W

linked to showing only one front 02 sensor.[...]

Good point. Well, it was supposed to be for the H6!

[Skip]

Thanks again OB99W

the power of positive reading,

I missed this

until you pointed it out.

 

18. Diagnostic Procedure with Diagnostic Trouble Code (DTC) S048521

A: DTC P0031 — BANK #1 AND SENSOR #1 OXYGEN (A/F) SENSOR

(FRONT RH) HEATER CIRCUIT LOW INPUT

[OB99W]

Does anyone find fault in me starting with this

proceedure?

I don't see a problem, other than possibly having a bad connection due to corrosion at the sensor connector go into "hiding". Since you're pulling the connector, why not just start with #9, and check the heater resistance. If it's open, your troubleshooting is completed.

 

Measuring voltage as in step #8 is best helpful if there is current being drawn. Any problem that's affecting voltage only to the front/right heater (and not the other two heaters) probably won't cause a noticable voltage drop on the voltmeter (most have a high input resistance and consequently draw little current). Still, it's certainly worth measuring.

[Skip]

check and double check,

will do

I didn't see #10?

"Since you're pulling the connector, why not just start with #10,"

 

I was hoping to find the a way to check the

heater resistance from topside.

 

Getting under the car at this time of year is somewhat

problematic.

 

Shame on me... I have a grease pit in my shed but the door is currently

blocked by my TT.

:^(

[OB99W]

check and double check,

will do

I didn't see #10?

"Since you're pulling the connector, why not just start with #10,"[...]

Sorry, there are only nine steps -- you saw my post before I noticed my error. I did edit and correct before you posted, however (11:39 vs 11:45) -- does that count?

[OB99W]

[...]I was hoping to find the a way to check the

heater resistance from topside.[...]

Well, that's why step #3 is a test procedure for checking the heater current draw. If the current is low, it's likely that the ECU is making the ground, but that there are other poor connections in the heater circuit. If it's zero, either the heater or wiring are completely open, or the ECU isn't making the ground. I believe that when the heater is on, it normally should draw about one-two amps. The "0.2 A" referred to in step #3 would indicate a fairly high resistance in the circuit (60 ohms or so including the heater), and even with a poor connection I'd expect "more than" that current.

 

Of course, step #3 requires either the Select Monitor, or an OBD-II scanner that has the feature.

 

If you could get to the main relay and ECU terminals, you could measure resistance from the main relay to the appropriate pin(s) for the front/right heater at the ECU, but that seems about as annoying as getting under the car.

[OB99W]

Good point. Well, it was supposed to be for the H6!

Skip, take a look at page WI-148 (not -138, that's my typo). It'll show both front O2 sensors.

 

(And why they put the 4-cyl info in the H6 supplement is beyond me, but...)

[WJM]

Does anyone find fault in me starting with this

proceedure?

 

Yes.

 

Quit wasting time diagnosing it and replace the A/F sensor.

 

Any P0031/P0171 type code relating to an A/F sensor heater, Lean/Rich code and the car seems to drive fine and the fuel trims are +/- 4% at idle...it means the A/F sensor needs to be replaced.

 

Trust me, I've chased my way around for about 4+ hrs diagnosing it only to find that everything tests 'good' and 'acceptable' then only to see that under WOT (with a P0171) it reports full lean (1.3+ Lambda) when in fact the actual reading is 0.72 lambda.

 

I replaced the A/F sensor as a 'it must be bad since its reporting like that' and whamo, fixed.

 

The next time I got one in...I spent 2 mins getting pricing and labor, sold it to the customer and boom it was fixed.

 

With the heater codes, I've never found any way to diagnose the A/F sensor bad *quickly* and with 100% certainty.

 

With rich/lean codes on a stock car in good running condition I've NEVER been able to diagnose and find any factual data that means the A/F sensor is truly bad....other than road load conditions with an Innovate (or AEM or other high quality aftermarket standalone wideband sensor setup) backing me up double checking what the car is doing. Then, boom, A/F reports either full lean or full rich for no reason, wideband reports the car trying to go more rich or more lean, fuel trims pass 25% for too long and the code sets.

 

In order to diagnose the A/F sensor the normal way, you'd have to be under hood at the A/F connector with a DVOM at the EXACT moment that it happens in order to have that info. Good luck with that...you'd need a road loading dyno and other expensive stuff to catch it.

 

Anyhow...

 

22641AA03C is what you need.

 

$90 dealer cost, $150 list. It would take me about 5 mins to install providing the threads dont self destruct on the way out.

[OB99W]

Quit wasting time diagnosing it and replace the A/F sensor.

22641AA03C is what you need.

[...]

$90 dealer cost, $150 list. It would take me about 5 mins to install providing the threads dont self destruct on the way out.

There you go, Skip. It's always good to know which part to throw at a problem.

 

I find it interesting that one dealer tech or another will have what they evidently believe is the definitive answer to a problem, but not speak up until one or more forum members try their hardest to help. If you've got the answer, why wait?

 

Sorry if I was "wasting" anyone's "time" (including my own).

[Skip]

OB99W I thank you again for your time and it

is appreciated on this end.

 

I am sorry for the responce delay, pretty half and I were doing the

Xmas thing. :^)

 

I don't know why WJM went on some tangent

in ref to the other error codes as I didn't mention,

read, or see them on my scan tool.

 

I printed the page WI-138 and thanks it gives me

the wire colors.

 

I need to research some more to find where

connectors E3 an E49 are, I can measure the resistance of

the heater at those connectors according to the page you listed.

 

Is the connector shown maybe the through the bulkhead connector?

 

Thanks again for the pages I will continue to endeavor to

persevere being cautiously optimistic as I do.

 

 

 

 

I think we both know I am not a throw parts at it kind of guy

thus the posting , this follow up and many thanks...

but I do think it is the 02 sensor as stated.

thanks WJM for the part number

 

[WJM]

I visit a few times a week...to make sure I dont miss a thread that was replied to and/or PM's....sometimes I dont get email notification. I do this on many forums, but I do not actively post or read any other part of the forum.

 

I was bored today and hit up the new posts button to see whats happening today....and found this thread and 3 more of interest.

 

Either way...I'm not one of those types to throw a part at a problem...I research it to death to make sure I'm going to fix the problem CORRECTLY the FIRST TIME. So those first 4+ hrs I spend on the P0031 code means everytime i get a P0031 it gets a new sensor. It has always fixed it and I've done that on at least 100 cars by now.

 

Reason why I mentioned the P0171 is because it sometimes comes in with the P0031 at the same time...most techs that I come across start replacing ECU's, wiring harnesses, main fuse boxes....etc etc...because it cant be the sensor! Its the dumb F.I. system! *rolling eyes so far back into head they get stuck*

 

Again, there was a car that I spend about two whole days on diagnosing a P0171 (System too lean). pressure testing, checking for intake leaks up-down-left-right-sideways-headassplosion, smoke testing the entire car and evap system, swapping out MAFs, MAPs, ECUs....then I swapped out the A/F sensor and the P0171 was all of a sudden on the car that A/F sensor was swapped to.

 

So...every P0171 has gotten an A/F sensor. Again, over 100 cars on that one and no comebacks.

 

I learned the hard way again on GM cars...P0172 (system too rich) with Zirconia O2 sensors (the normal type O2 sensor found on ALL rear locations for 2000+ SUBARUs, and the FRONT on everything up to 1999 EXCEPT Calif. MY99 SUBARUs)...it just means replace it and let it roll. Again, no come backs on that one...however the count is low, less than 10 cars, on seeing a P0172.

 

Another factor is that I contribute when I see that people are being intelligent about the situation. If people are being complete idiots...well, no use for me there because 100% of the time they argue back that I am wrong and dont know what I'm talking about. No need to waste my time there.

 

Anyhow...I'll /tanget now. lol

 

And ya'll are welcome btw. Merry xmas!

[OB99W]

[...]I think we both know I am not a throw parts at it kind of guy

thus the posting , this follow up and many thanks...

but I do think it is the 02 sensor as stated.

thanks WJM for the part number

You're quite welcome, Skip. I don't think you're the type to just assume a part is bad, and I didn't at any time say the O2 sensor wasn't bad; it's distinctly possible (likely?) that it is.

 

Although WJM's post triggered my comments, his information is certainly useful. In general, it may well be that a particular OBD trouble code on a particular model almost always has the same cause, and that an experienced tech will on average save the customer money by just replacing that part rather than doing lengthy diagnostics.

 

It's just that I assume many (admittedly, not all) folks using the forum are looking for a diagnostic approach, in order to feel confident that they're not buying an unneeded part (that some may not be able to afford). Another consideration (that WJM's "providing the threads dont self destruct on the way out" brings up) is whether efforts to remove a part will result in a great struggle, cuts/abrasions, and swearing. I hope I'm correct, if that's the case, that people will be willing to spend some time doing the diagnosis. I also sometimes include info in my posts that goes beyond what's needed by the OP, with the thought that others searching later might find it useful. Moving on...