OBD-II trouble code
P0171: System Too Lean (Bank 1)
The engine computer detected that Bank 1 is running with too much air or too little fuel. The check engine light is on and you may notice rough idling or hesitation, but the car is usually still drivable.
Quick facts
- System
- Powertrain
- Category
- Fuel and Air Metering
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $1,000
- DIY difficulty
- Intermediate DIY
What does P0171 mean?
The engine control module (ECM) constantly monitors the air-fuel mixture using oxygen sensors and adjusts fuel injection to keep combustion ideal. "Lean" means there is too much air relative to fuel — the opposite of a rich mixture. Short-term fuel trim (STFT) and long-term fuel trim (LTFT) record how much the ECM has had to add fuel to compensate. When LTFT climbs past roughly +10% to +25% for an extended period (the exact threshold varies by manufacturer), the ECM sets P0171.
Bank 1 refers to the side of the engine that contains cylinder number 1. On an inline engine, there is only one bank, so P0171 always applies. On a V6, V8, or other V-configuration engine, P0171 is specific to Bank 1, while P0174 covers Bank 2.
P0171 has many possible causes, and the most common ones — vacuum leaks and dirty mass airflow (MAF) sensors — are inexpensive to address. Less common causes like a weak fuel pump or leaking injectors can be more expensive. Diagnosing in order from cheap to expensive saves money.
Common causes
- Vacuum leak from a cracked intake hose, broken PCV valve, or intake manifold gasket (most common)
- Dirty or failing mass airflow (MAF) sensor
- Weak fuel pump or clogged fuel filter reducing fuel pressure
- Leaking or clogged fuel injectors
- Exhaust leak upstream of the oxygen sensor confusing the ECM
- Faulty upstream oxygen sensor reading lean
- Stuck-open EVAP purge valve drawing unmetered air
- Air filter housing not seated properly after recent service
Symptoms
- Check engine light is on
- Rough or unstable idle
- Hesitation or stumble on acceleration
- Occasional misfire under load
- Reduced fuel economy
- Possible stalling at idle in severe cases
Diagnostic steps
- 1.Scan the vehicle and record freeze frame data plus current short-term and long-term fuel trims.
- 2.Inspect intake hoses, the PCV valve, the brake booster hose, and the intake manifold gasket for cracks or loose connections.
- 3.Perform a smoke test or use carburetor cleaner around suspect joints — a temporary RPM change indicates a leak.
- 4.Inspect and clean the MAF sensor with MAF-specific cleaner. Test scan-tool MAF grams per second against the manufacturer's specification at idle.
- 5.Check fuel pressure at idle and under load against the manufacturer's specification.
- 6.Inspect the upstream oxygen sensor signal pattern — slow switching or stuck-lean readings indicate a worn sensor.
Repair cost
$50 – $1,000
Range is wide because the fix could be a $30 vacuum hose, a $150 MAF sensor cleaning or replacement, or a $400-$900 fuel pump replacement. Diagnose vacuum leaks and the MAF first — they account for the majority of P0171 cases.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with vacuum leak repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.