System Too Lean Bank 1
What it means
P0171 means the engine control module has detected too much air relative to fuel on bank 1 — the mixture is running lean and fuel trims have hit their correction limit. It is usually caused by an unmetered air leak or a MAF or fuel-delivery problem, and left unfixed it can trigger misfires.
Can I keep driving? Usually drivable, but a persistent lean mixture can cause hesitation, rough idle and, if left unfixed, misfires that overheat the catalytic converter — repair it soon.
Real-world scan data
Appears alongside · share of P0171 scans
Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.
Common symptoms
What you'll notice
- Check-engine light on
- Rough idle, hesitation, or occasional stalling
- Reduced power and slower acceleration
- Slightly worse fuel economy or a lean surge at cruise
Common causes
Most likely first
- Vacuum or intake leak letting in unmetered air (most common)Likely
- Dirty or failing mass air flow (MAF) sensor reading lowCommon
- Low fuel pressure — weak fuel pump or clogged fuel filterCommon
- Clogged or dirty fuel injectorsLess often
- Exhaust leak near the upstream O2 sensor skewing readingsLess often
- Failing upstream oxygen sensorLess often
How to diagnose & fix
Work through it in order
- Read freeze-frame and long/short fuel trims to confirm a true lean condition, then smoke-test the intake for vacuum leaks.
- Inspect intake boots, PCV and vacuum hoses, and the intake gasket for cracks or loose connections.
- Clean or test the MAF sensor and check for an unmetered air path after it.
- Check fuel pressure against spec; service the fuel filter and clean injectors if pressure or delivery is low.
- Inspect the exhaust manifold and upstream O2 sensor area for leaks, then clear and re-test.
P0171 by make
Based on commonly reported failure patterns across owner and repair communities, not on our scan data.
BMW
Commonly reported on BMWs from crankcase ventilation parts, cracked intake boots and vacuum hoses that harden with age. These leaks are often cheap to fix once found.
Chevrolet
Common on Chevrolet trucks with 5.3L and 6.0L V8s, where intake manifold gasket leaks and vacuum leaks are frequent causes. A dirty MAF sensor is another cheap thing to check.
Ford
Commonly reported on Ford V8 trucks such as those with the 5.4L Triton, where intake gasket and vacuum leaks are known causes. A dirty MAF sensor can also add to it.
GMC
Common on GMC trucks with 5.3L and 6.0L V8s, where intake manifold gasket leaks and vacuum leaks are frequent causes. A dirty MAF sensor is another cheap thing to check.
Honda
Commonly reported on Hondas from a PCV valve or hose that has gone bad, a vacuum leak, or a dirty MAF sensor. Inspect the rubber hoses first.
Jeep
On Jeeps, vacuum leaks and cracked intake tubing are common causes, and the 4.0L inline-six is commonly reported to develop intake and vacuum leaks as it ages. Smoke-testing the intake finds these leaks quickly.
Nissan
Commonly reported on Nissans from vacuum leaks, intake tube splits or a dirty MAF sensor. Look over the intake tubing for cracks before spending on parts.
Toyota
On Toyotas, a failing PCV valve or hose, or a small vacuum leak, is a commonly reported cause. A dirty MAF sensor can also push the mixture lean.
Volkswagen
On VW and Audi turbo engines, cracked PCV or crankcase breather parts are a well-known source of unmetered air. Check these before replacing sensors.
FAQ
Quick answers
Is it safe to drive with P0171?+
Can I just clear P0171?+
Related codes
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