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Old 05-28-2013, 11:40 AM   #1
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Join Date: May 2013
Location: Los Angeles
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Need help with how to proceed when throwing only codes P0171 and P0174 together.

Hi Guys.

2003 Boxster S. 78K. Check engine light came on for first time.
Code reader gives me P0171 and P0174. Those two codes only.
P0171 = fuel system too lean (bank 1); P0174 = fuel system too lean (bank 2).

Did a search here on the forum and found surprising little relating to these specific codes firing together only.

The best generic write-up I can find is a Repair Pal post, here:


Article suggests that it wouldn't be O2 sensor related, as it would be very unlikely for both sensor sides to foul at once. Makes sense to me.

It seems that the two most likely problems are:

1) the MAF sensor (dirty and/or faulty) (clean and do a barometric pressure scan test)
2) a vacuum leak somewhere (intake manifold gasket, vacuum hoses, PCV hoses, etc.) (visually inspect and possibly smoke test)

It seems that less likely culprits are:

3) out of date PCM software (update) (how do I tell?)
4) poor fuel delivery (failing fuel pump, clogged fuel filter, fuel injector failure) (test fuel pressure, check/change fuel filter, test injectors)

Can any of you Porsche experts out there tell me if I'm on the right track, or give me further valuable insight into this throwing this pair of codes only on a Boxster? ie. What's the common failure? What should I check first? What can I deduce? In what order should I proof things out? Etc..

Thank you!

*** POSSIBLY WORTH NOTING -- I AM NOT EXPERIENCING ANY NOTICEABLE CHANGE IN THE CAR'S PERFORMANCE. IT SEEMS TO BE IDLING AND RUNNING AS WELL AS IT ALWAYS DID. [That makes me lean toward MAF (sensor related) rather than vacuum leak (physical lack of fuel or too much air). Though I suppose it may be that the car's computer is compensating well enough for whatever physical air/fuel problem may be.. so perhaps my reasoning is faulty there..]

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Old 06-05-2013, 11:23 AM   #2
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Join Date: May 2013
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Found the problem, guys!

Not that anyone responded.. :/

It was the AOS. It's diaphragm had begun to tear. Was sucking air into intake and creating a strong crankcase vacuum, substantially above what it should have been. Could barely budge the oil filler cap when the car was running.

Fortunately (and amazingly) there was no oil yet in the intake, so I was safe. And lucky.

Replaced the AOS, and now all the parameters (including fuel trim) are back within spec and the car is running perfectly. DO notice a difference in performance.

Figured I'd post, just in case it might help anyone else down the line.

Now I just have to change the oil and swap the engine mount out, and she'll be all fixed up.

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