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2009 Outback 3.0r, 1998 Forester
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39 Posts
Discussion Starter · #1 ·
Hi, I could use a little help. Car is 2002 H6 UK spec, 70k miles.
CEL came on, along with 'VDC off' light. I pulled a P0151 code: O2 sensor circuit low voltage bank 2 sensor 1. I've done the obvious and checked the connection from the front O2 sensor to the loom, and checked for vacuum and exhaust leaks. Is this likely to be as simple as a sensor replacement, or are darker forces at work here, and why the 'VDC off' light? It clears if I stop the engine and restart. There's only the one code, but I could log the fuel trim values if that would be useful. Any suggestions as to what the problem might be are most welcome!
 

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01 Outback H6 VDC, 97 GT wgn w/ ej22, 98 OBW w/ej22
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if the uk h6 is like mine in the us, there is a dual exhaust leaving the engine and a cat or cats in each. then they join around the rear of the trans. there are o2 sensors in front of and behind each cat on each side of the exhaust and a fifth one behind the joined exhaust.

the point is, bank 2 sensor 1 is the passenger side front o2 sensor.

i would double check the wiring and connectors. low circuit, or i assume low voltage, is probably a wiring problem. if the heater was bad the code would say that. if the readings were wrong it would indicate the cats were not working as they should. this is an educated guess and could be wrong, but it is my best guess.

good luck.
 

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2009 Outback 3.0r, 1998 Forester
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39 Posts
Discussion Starter · #3 ·
Thanks, as stated, I've checked all the basics, no problems found, so I've ordered a Denso oem replacement at reasonable cost. I'm just curious as to why a non-critical code like this would switch off the VDC.
 

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01 Outback H6 VDC, 97 GT wgn w/ ej22, 98 OBW w/ej22
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my experience is limited, but ...

the vdc will turn off and turn on the ''off'' light under lots of unfavorable conditions. i have read that bad gas/ petrol can cause it, or overheating will kick it off. just 2 examples.

and i know for sure that when driving on a 2 lane hiway and you punch it to pass, if you go fast enough for long enough, the AC will shut off. this is not VDC specific, but it does support the idea that the computer will turn things off for almost any fault. the VDC being one of them.

another example, any time the computer throws a Check Engine Light, it will also disable the cruise control. the point being, they do not want you to ignore the CEL.
 

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2009 Outback 3.0r, 1998 Forester
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39 Posts
Discussion Starter · #5 ·
Ok here's an update:
VDC off light always comes on when CEL lights, according to the manual, so nothing amiss there.
Still throwing P0151 code, (bank 2 upstream sensor low voltage).
Scanner shows O2 voltage for both bank 1 and 2 fluctuating correctly, giving similar figures. At idle, short term fuel trim for both banks is about equal, and fluctuates along with O2 sensor voltage, as it should. However, at 2500 rpm, short term fuel trim on bank 2 is way higher than bank 1, more than twice as high. This seems to be telling me that bank 2 is running lean. Since the short term fuel trim rises at higher revs, it's probably not a vacuum leak, but perhaps low fuel pressure on bank 2? I would appreciate some input into this, as information on the H6 is a bit thin on the ground here in the UK. Thanks!
 

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Premium Member
(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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any work done recently? it would be quite easy to hit that sensor or its wiring with a wrench during a plug or valve cover gasket change.

If you clear the code does it return immediately?

is the battery in good condition?


just some ideas - it seems like you have good toubleshooting skills and some equipment so, I may learn something from this thread.
 

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2001 H6 OBW
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97 Posts
With my H6, I had problems with the EVAP system. A bad solenoid. This triggered the CEL. I still have EVAP trouble code pop on from time to time and the CEL and VDC Off light seem to always come on together. I don't recall seeing the CEL withouth the VDC Off light.

... after reading the rest of this post, I see that someone already confirmed this. :29:
 

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2009 Outback 3.0r, 1998 Forester
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39 Posts
Discussion Starter · #8 ·
Hey thanks for the responses. Seems I may have fixed it. The replacement sensor arrived in the post this morning, so I fitted it, cleared the code, and no CEL after 50 miles of mixed driving (it came on less than a coupe of miles before). I'll see if it comes back. Throttle response is much improved, and the car reacts to kick down much faster. I was trying to avoid just chucking an expensive part at the problem, and learning more about how to interpret diagnostic info. My short term fuel trim on bank 2 is now similar to bank 1. Unfortunately by clearing the code, the memory is also cleared, which includes the long term fuel trim data, which can be very useful in conjunction with other info. What is confusing to me is that the bad sensor was generating the correct voltages, and fluctuating at the right rate. Maybe it had just got a bit slow to react with age. A bit like me......
Anyway, what I've learned is that a P0151 code does not necessarily mean a bad sensor (although in this case it might be), and it's important to read the fuel trims to get a fuller picture of what's going on. There's loads of good stuff on YouTube about all this. Also I've now checked all my vacuum hoses, and cleaned the PCV.
 
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