Drivable short-term. Diagnose promptly to avoid engine damage.. P0171 means the engine management system is adding maximum fuel correction but the air-fuel mixture on Bank 1 is still running leaner than the target 14.7:1 ratio.
What P0171 means
Fuel trim is the ECM's continuous correction to the base fuel map. Short-term fuel trim (STFT) adjusts in real time; long-term fuel trim (LTFT) is a learned, persistent correction. When Bank 1 needs more than roughly +25% STFT for a prolonged period, the ECM concludes that the base calibration cannot compensate for the lean condition and stores P0171. 'Bank 1' refers to the cylinder bank containing cylinder number 1 — on 4-cylinder engines there is only one bank, so P0171 is the only lean code possible. On V6 and V8 engines, a companion P0174 indicates Bank 2 is also lean.
Symptoms
- Check engine light on (may pulse during idle on severe lean conditions)
- Rough or unstable idle, occasionally with stalling
- Hesitation or stumbling under light acceleration
- Popping or backfiring sound on deceleration in severe cases
- Elevated fuel consumption as the ECM over-fuels trying to compensate
- Long-term fuel trim (LTFT) above +10% visible on a scan tool
Common causes
- Vacuum leak — cracked intake manifold gasket, split hose, or PCV system breach that introduces unmetered air (most common cause)
- Dirty or failing mass airflow (MAF) sensor reporting lower air mass than is actually entering the engine
- Weak or clogged fuel injectors on Bank 1 cylinders delivering less fuel than commanded
- Low fuel pressure from a failing fuel pump, clogged fuel filter, or faulty pressure regulator
- Faulty Bank 1 upstream oxygen sensor stuck in a lean-reporting state
- Exhaust leak near the upstream O2 sensor pulling in ambient air and biasing the reading lean
Severity & driving advice
Severity: Moderate — Persistent lean mixture raises combustion temperature, accelerating wear on pistons, valves, and the catalyst. Diagnose within a few weeks.
Can I drive? Drivable short-term. Diagnose promptly to avoid engine damage.
Diagnostic approach
- Monitor short- and long-term fuel trims with a scan tool — Connect a scan tool and read STFT and LTFT for Bank 1 at idle and at 2,500 RPM. LTFT above +10% confirms the ECM is compensating for a lean condition. If trims worsen at idle and improve under load, suspect a vacuum leak. If trims are consistently high regardless of RPM, suspect a fuel delivery issue.
- Inspect and clean the MAF sensor — A contaminated MAF hot wire or film sensor under-reports air mass, causing the ECM to under-fuel. Remove the sensor and clean the sensing element with dedicated MAF cleaner spray (never generic carb cleaner). Allow to dry fully before reinstalling.
- Smoke-test the intake system for vacuum leaks — Introduce low-pressure smoke into the intake downstream of the MAF sensor (throttle body area or intake manifold) with the engine off. Smoke emerging from gaskets, hoses, or the PCV system confirms unmetered air entry. This is the most reliable method to find intermittent leaks.
- Check fuel pressure at the rail — Using a fuel pressure gauge, verify pressure meets specification at idle and under snap-throttle. Low or dropping pressure after the pump shuts off points to a failing pump, weak check valve, or pressure regulator fault.
Typical repair costs
| Component | Low estimate | High estimate |
|---|---|---|
| MAF sensor replacement | $80 | $250 |
| Vacuum leak repair (hose, gasket) | $30 | $200 |
| Fuel injector cleaning (all cylinders) | $100 | $200 |
| Fuel pump replacement | $300 | $700 |
| Upstream oxygen sensor (Bank 1) | $80 | $250 |
Make & model notes
Toyota: The 1MZ-FE V6 (Camry, Avalon, Sienna) is notorious for cracking intake manifold runner control gaskets as the plastic ages, creating large vacuum leaks. Inspect the lower intake gaskets before assuming the MAF is at fault.
Ford: Ford 4.6L and 5.4L Modular V8 engines frequently develop P0171/P0174 from split or collapsed PCV system hoses behind the intake manifold. These hoses are inexpensive but awkward to access, especially on the Mustang GT and F-150.
BMW: The N52 and N54 inline-6 engines develop P0171 from cracked intake boot couplers (the rubber accordion section between the MAF and throttle body) and deteriorated valve cover gaskets that allow oil vapour to foul the MAF sensor.
FAQ
What does 'Bank 1' mean in P0171?
Bank 1 is the side of the engine that contains cylinder number 1. On inline 4-cylinder engines, there is only one bank so P0171 is the only lean code. On V6 and V8 engines, Bank 2 (the opposite bank) has its own lean code: P0174.
Can a dirty air filter cause P0171?
A severely clogged air filter can restrict airflow enough to cause a rich condition, not lean. However, a degraded or incorrectly seated air filter element can allow unmetered air to bypass the MAF sensor, creating a lean reading. Check that the filter housing is sealed properly.
My fuel trims are high but I cannot find a vacuum leak — what else should I check?
Look at the fuel injector pulse width data on a scan tool. If the ECM is commanding long injector pulses but the fuel trims are still high, suspect weak injectors or inadequate fuel pressure. Also check the oxygen sensor response time — a slow or lazy O2 sensor can generate false lean codes.
Will P0171 cause me to fail an emissions test?
Yes. P0171 sets a readiness monitor as incomplete or failed, and most OBD-II emissions programs will fail a vehicle that has an active DTC. Fix the fault and complete the required drive cycle before testing.