What P0171 means
The oxygen sensors report the mixture is running lean — more air than the injected fuel accounts for — and the computer has maxed out its correction trying to compensate. Almost always this means unmetered air is sneaking in (a vacuum leak) or the airflow sensor is under-reporting, not that fuel is genuinely short.
What you’ll notice
- Check engine light
- Rough idle that smooths at higher RPM
- Hesitation or stumble on acceleration
- Sometimes idle surging
Likely causes and repair cost
Broad US independent-shop bands, parts and labour.
Can you drive with P0171?
Drivable in the short term; sustained lean running raises combustion temperatures and, over time, risks damage to pistons and valves. Weeks of margin, not months.
The misdiagnosis to avoid
Replacing oxygen sensors because they're the components in the story. The sensors are the reporters, not the fault — they measured lean correctly. The classic P0171 fix is a $30 intake boot found in a $90 smoke test, discovered after $400 of sensors changed nothing.
Describe what it’s doing in your own words and the symptom checker gives you a safe-to-drive verdict, ranked causes with costs, and notes to take to the shop — free, no sign-up.
Read the code yourself
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Best for: Anyone who just wants to know what the light means
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Best for: People who want live data and app flexibility
Check price on AmazonQuestions people ask
What's the cheapest first move on P0171?
Clean the MAF sensor with dedicated MAF cleaner ($10) and inspect the big intake tube between airbox and throttle for cracks — flex it open with the engine running and listen for idle change. Those two DIY steps resolve a genuinely large share of P0171s.
Why does it idle rough but drive fine?
A vacuum leak is a fixed-size hole; at idle it's a large fraction of total airflow, at cruise it's a rounding error. Roughness that vanishes with RPM is close to a vacuum-leak signature — and the smoke test is the direct path to the hole.
P0171 and P0174 together — what changed?
Both banks lean at once shifts suspicion to what's shared: a single large vacuum leak (intake manifold, brake booster hose), the one MAF sensor, or overall fuel pressure — rather than a per-bank fault. Paired lean codes narrow the search usefully.