P0171 — System too Lean (Bank 1)
Category: Powertrain Severity: Medium
Updated:
DTC code P0171 means: System too Lean (Bank 1). It falls under the "Powertrain" category, with an estimated severity of Medium. Common causes include vacuum / intake leak, faulty MAF sensor and low fuel pressure; it's worth confirming the diagnosis before replacing parts.
Possible causes
- Vacuum / intake leak
- Faulty MAF sensor
- Low fuel pressure
- Faulty O2 sensor
- Clogged fuel injector
Symptoms
- Loss of power when accelerating
- Unstable idle
- Higher fuel consumption
- Jerking while driving
How to diagnose it step by step
- Read fuel trims at idle and at 2,500 rpm.
- If they improve with rpm, look for vacuum leaks (smoke test).
- If they worsen with rpm, check the MAF and fuel pressure.
- Clean the MAF with dedicated cleaner.
- Check the PCV valve and brake booster hose.
How to fix it
Clean or replace the MAF/MAP sensor and check for intake leaks.
Estimated repair cost
Vacuum hose: $10–60. MAF cleaning: $20–70. MAF sensor: $100–300. Fuel pump: $300–800.
Frequently asked questions
What does DTC code P0171 mean?
System too Lean (Bank 1).
Is code P0171 serious?
Severity Medium. Not urgent, but worth diagnosing to avoid bigger damage.
Can I keep driving with code P0171?
Yes, short term. A prolonged lean mixture can cause misfires and overheat the catalytic converter.
Related codes
P0172 P0174 P0175 P0161 P0157 P0156 P0155 P0153 P0190 P0151
Refine by car make
- Audi P0171: Vacuum leaks or mass air flow (MAF) sensor.
- BMW P0171: Intake leaks and at the PCV valve gasket.
- Citroën P0171: MAF/MAP sensor or leak.
- Dacia P0171: Same as Renault: intake leaks or MAP/MAF.
- Fiat P0171: Intake leaks or MAP sensor.
- Ford P0171: Vacuum leaks and a dirty MAF sensor.
- Honda P0171: Vacuum leaks or MAF/MAP sensor.
- Hyundai P0171: Vacuum leaks or MAF sensor.
- Kia P0171: Vacuum leaks or MAF sensor.
- Mazda P0171: Vacuum leaks or MAF sensor.
- Mercedes-Benz P0171: Intake leaks or MAF sensor.
- Nissan P0171: Vacuum leaks or MAF sensor.
- Opel P0171: Vacuum leaks or MAF sensor.
- Peugeot P0171: Intake leaks or MAF sensor.
- Renault P0171: Intake leaks and MAP/MAF sensor.
- Seat P0171: Same as VW: vacuum leaks or MAF.
- Škoda P0171: Same as VW: vacuum leaks or MAF.
- Toyota P0171: Dirty MAF sensor or vacuum leak.
- Volkswagen P0171: Vacuum leaks and mass air flow (MAF) sensor.
- Volvo P0171: Vacuum leaks or MAF sensor.
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