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Discussion Starter · #1 ·
Hello All. I am new to the forum, and joined partly from desperation. We have a 2008 Tribeca with 160k miles. It has been a great vehicle and still drives beautifully. However, in the last five months we have had sensors fail on us at least four times requiring expensive repairs. Last October it was a mass air flow sensor. In December it was an oxygen sensor (I partially recall but have misplaced the paperwork). Then in the last two weeks we have replaced both the Bank 1 and Bank 2 oxygen (air-fuel ratio) sensors. Of course, each time we lose cruise and traction control. Well my wife just saw the engine light come on again this evening while driving across the PA Turnpike. Is this unusual? How many more will we need to replace? Is there an underlying problem causing all of these sensors to fail in a short time? I appreciate any advice or suggestions
 

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Welcome to the forum. I can't really address this issue, as I have not had this happen. Granted, I only have 156k, and I have the 3.0, not the 3.6, but I have not had any sensors fail. Maybe others will chime in with their experiences. (I have a 2007 Tribeca)
 

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Discussion Starter · #3 ·
Thanks for the welcome SubieDriver. I have looked on line for a list of the suite of sensors that might be subject to further failure. But I could not easily locate that list. This long episode began right after we had a Smog Test run. So it was a double poke-in-the-eye. Maybe someone else will chime in that these failures are reasonable at ~160k miles.
 

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These failures are not reasonable at 160k, and I wonder if they are 'hard failures' or if your mechanic is 'throwing parts at it.'

Can you tell us the code? Hopefully each time a repair was made, a code was pulled and documented.

I suspect it's P0420 but maybe not.

Another possibility is bad grounds, bad battery or bad alternator. Low voltage has been the cause of a phantom codes before. Has the alternator ever been replaced? With what? (there is a common issue with aftermarket alternators nor providing proper charging).

Any other repair and maintenance history you can give us?
 

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You could also try posting over in the Nasioc Tribeca forum. I don't see any recent posts there, but it has been active in the past. On the forums page, it's near the bottom.
 

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I have absolutely no empirical evidence to back this up, but when I see MAF, knock, fuel mix, coolant temp, MAP, and/or O2 sensors failing in sequence or coincidentally, I immediately think battery/voltage first issues and fuel/additive issues second before I'd even guess as to whether or not the sensors were actually bad.

Another thing to consider... are the diagnoses and replacements being done at a dealership? Once upon a time, I had a 1990 Sunbird that had a bad IAT sensor... I replaced the thing with a crappy Autozone part about 6 times in two years (making a warranty claim every time); one day, I went to the dealership, got an OEM sensor for $10 more than AZ, and never had the problem again.
 

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Discussion Starter · #8 ·
tdelker, I was also wondering if there was not a separate issue causing these "failures" In fact. More than one have been P0171 codes. In fact, just found out that one sensor that was replaced in PA, was replaced 2 months later by a Subaru dealer in Indiana.

You mention alternator problems. We have had a long, long history of replacing headlights on a regular (once a year) basis. Might we be having a voltage regulator problem?
 

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Discussion Starter · #9 ·
psygnall11, you also mention the voltage problems. I just mentioned to tdelker that we have had a long history of replacing headlights on a very regular basis. Might a voltage regulation issue be causing either burnt out sensors or false readings?

As for the parts, two have been dealer repairs, and two independent shops (spread across the country). All the parts costs have been PREMIUM (~$300 for a sensor).
 

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Discussion Starter · #10 ·
tdelker, just spent a few minutes looking up some of the codes. The initial problem solved with a replaced mass air flow sensor. Since then it has been P0171, P0172 and P0175 issues.
 

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The headlight bulb deal seems to be fairly common to the Tribeca, or the 08 on up anyway. Hasn't been an issue with my 07 nor with the 1st owner.
If o2 sensors and af sensors are being replaced then it sounds like a P0420 code as tdelker suggested. I think it may have been him that gave me a link to a thread on the outback forum. It was 90 some odd pages of reading, but I fixed mine by cleaning the ground cable and contact points. Only money I spent was for a new pcv valve that I am sure needed replacing with 160 thousand miles on the original. That thread is invaluable. Get this latest code read and get back to us.
 

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Discussion Starter · #12 ·
Jefferson. Thanks. I will be getting up early here on the west coast and call the Philly area for the independent shop doing the service now. They read the code today, but I forgot already what the number was and will get it in the morning.

Which ground cable and contact points to which device? Sorry for being thick here, but I am still trying to get a sense of how many sensors this vehicle has. By the way, are there BOTH O2 sensors AND air-fuel sensors on this car?
 

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The main ground cable from the battery to the engine. There was a 3rd end that went to the fuse box I think.
There are 2 types of sensors and from the outback thread I learned that one of them has to be a Denso. Don't use a Bosch.
If it is a P0420 code it could be caused by a myriad of things from poor fuel, dirty injectors, thermostats, electrical and so on.
 

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Discussion Starter · #14 ·
Jefferson, Thanks. I am still trying to get paperwork details on the particulars of the sensors that were replaced thus far. One mass air flow sensor in October, one O2 or A/F sensor (P0171 code) in December, a throttle air box in Feb (P0172 and P0175), a bank 1 O2 sensor (P0171) in early March and a A/F sensor last week. Now I or they may be mixing O2 and A/F sensor names. Still waiting to see tomorrow what the current fault reads.
 

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I don't know what those codes refer to, but it sounds like there is a problem somewhere that they haven't found and are chasing things.
 

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Discussion Starter · #16 ·
Yeah, it is feeling that way.

P0175 = Bank 2 is running rich
P0172 = Bank 1 is running rich
P0171 = bank 1 is running lean

Sigh.
 

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Sounds like an exhaust leak to me. Anyone else?

Or the wrong spark plugs. ONLY what is called for in the owner's manual, NGK brand, exact model.
 

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Discussion Starter · #19 ·
tdelker, the current OBD code is P0171 (Bank 1 A/F too lean). This is the same code that has come up several times in the past and we have had a Bank-1 O2 sensor changed twice in the last 3 months. My wife has it at a local service station (not a Subaru dealer). They want to look for air box leaks and then the fuel injectors. I passed on the info that the Mass Air Flow (MAF) sensor had been changed at the beginning of this long saga. It appears that the lead tech is not going to just replace a part and is trying to find the source of the problems. Help here is appreciated.
 

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For P0171-P0175, the diagnostic procedure is the same, in order:

Check exhaust for holes, loose bolts, etc.
Check intake for holes, loose bolts, hose disconnection, etc.
Check fuel pressure
Check engine coolant temp sensor
Check MAF air flow data
Check MAF temp vs. ambient
 
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