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Check Engine Light Problems

9K views 22 replies 5 participants last post by  Surprise28 
#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
 
#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.
 
#4 ·
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?
 
#6 ·
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.
 
#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?
 
#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).
 
#11 ·
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.
 
#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?
 
#13 ·
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.
 
#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.
 
#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.
 
#20 ·
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
 
#22 ·
No problem... one item to clarify is that I summarized the procedure by saying "check," i.e. when I say to "check" the coolant sensor, I don't mean simply check to make sure it's working, I mean check to make sure it's accurate.

Too many people - and mechanics - assume that as long as a sensor isn't triggering a fault or CEL, the sensor is working properly. Many sensors, like the coolant sensor, won't actually trigger a CEL unless it fails completely (i.e. loses its signal), or in a manner that makes it obvious to the ECU (e.g. gets stuck on a high or low signal). In other words, you can have a coolant sensor sending a signal that is off by +/-40 degrees, yet it might not trigger a CEL; however, it's going to throw off the ECU and could create problems elsewhere (in this case, a lean/rich fault).
 
#23 ·
psygnal11, I heard from my wife a bit ago. For the moment (and I say that with some uncertainty), the car seems to be back on the road. The service tech checked for leaks in the air intake system (none found), he adjusted (or checked) the fuel trim level, and then cleaned the fuel injectors. This seems to have been his plan from the outset. He then cleared the error functions and they did not re-trip. My wife took the car ... but we need to evaluate it over the next couple days. The last "fix" lasted 2 days and 500 miles. I sincerely hope that this corrects any underlying issues.

I will keep this thread updated on any further developments. For now, I would like to thank heartily those who gave advice. Thank you.
 
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