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So, your check engine light just came on
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and you pulled a P 0171 or a P 0174
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code, huh? Yeah, those two usually mean
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your engine's running too lean.
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Basically, it's getting too much air and
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not enough fuel. Hey, I'm Tom and
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welcome back to Car Justified. I'm a
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mechanic who's been elbow deep in Audi's
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and Volkswagens for way too many years,
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but I still love this stuff, and I'm
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here to help you fix it yourself without
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getting ripped off. All right, so P 0171
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These codes pop up a lot on VWs and
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Audi's, especially the ones with
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turbocharged engines. It's almost always
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something small causing a big headache.
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But don't worry, we're going to break it
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down step by step. And I'll even show
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you how a simple smoke test can make
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your life 10 times easier. Now, before
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you go swapping parts or spending a
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fortune, the first thing you want to
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know is these codes usually mean there's
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a vacuum leak somewhere. And trust me,
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even a tiny crack in a hose can throw
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everything off. First tip, check your
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intake hoses and vacuum lines. Pop that
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hood and start looking for anything that
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looks cracked, brittle, or disconnected.
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Don't forget to tug on the connections a
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little. I've seen hoses that look fine
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until you barely touch them and bam,
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they split wide open. One common spot,
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that PCV system. That's the positive
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crankase ventilation valve. On a lot of
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Audi's and VWs, especially the 2.0
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turbos, the PCV valve loves to fail. It
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can create a vacuum leak and mess with
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fuel trim at the same time. If your
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engine's whistling like a teac kettle or
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idling rough, that could be it. Yep,
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that's usually the culprit. Next up, the
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mass air flow sensor or MF for short. If
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it's dirty or faulty, it sends the wrong
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data to your ECU. That's the engine
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control unit, and that can trigger those
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lean codes. Just pull it out and give it
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a clean with some proper MAF cleaner.
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Don't go poking it with a screwdriver or
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anything, though. Those sensors are
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delicate, like grandma's china. Okay,
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but here's where the magic happens.
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Smoke testing. This is hands down the
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best way to find vacuum leaks, and it's
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easier than you think. You don't need a
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$1,000 machine, either. You can grab a
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cheap smoke tester online, or even build
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one yourself with a metal can, some baby
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oil, and a little air pump. I might do a
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video on that if you guys are
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interested. All you do is connect the
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smoke tester to your intake system,
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usually where the air filter goes, and
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lightly pressurize it. Then just watch
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anywhere the smoke starts leaking out.
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Bingo. You found your problem. I had a
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buddy with an Audi A4 that kept throwing
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that P 0171 code. We smoke tested it and
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found a teeny tiny split in the hose
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going to the brake booster. Replaced it
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for 15 bucks and the light's been off
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ever since. Another one to check is the
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intake manifold gasket. Over time, that
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rubber can get hard and start leaking
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around the edges. The smoke test will
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show it clear as day. If you see smoke
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puffing out around the intake, you know
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what's up. Now, let's not forget the
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fuel system. A weak fuel pump or a
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clogged filter can also cause a lean
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condition. Not as common, but if you've
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ruled out all the vacuum stuff and your
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fuel trims are still way off, you might
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want to test the fuel pressure next. All
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right, quick recap. If you've got a
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P0171 or P0174 on your VW or Audi, start
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simple. Check your vacuum lines, look at
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the PCV system, clean the MF sensor, and
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then run a smoke test to hunt down any
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leaks. It's way cheaper than throwing
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parts at it, and honestly, kind of fun
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in a weird, nerdy way. If this helped
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you out or saved you a trip to the
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dealership, go ahead and hit that like
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button and subscribe if you want more
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nononsense car fixes from yours truly.
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And hey, if you've got a weird issue or
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you're stuck, drop it in the comments. I
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read them and I'm happy to help when I
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can. Thanks for watching Car Justify.
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I'm Tom and I'll see you next time with
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more car stuff that actually makes