Fuel & air metering · Powertrain · generic (SAE-defined)
P0171: System Too Lean (Bank 1)
P0171 means the engine computer has reached the limit of how much fuel it can add on bank 1, because the engine is getting too much air or too little fuel.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0171 sets
The powertrain control module (PCM) adjusts fuel delivery using feedback from the upstream oxygen sensor and records the corrections as short term and long term fuel trims. When the engine keeps reading lean, the PCM adds fuel by pushing trims positive. If the total correction on bank 1 stays beyond its calibrated limit for long enough, the PCM concludes it can no longer compensate and sets P0171. The trim limits and enabling conditions are manufacturer-specific, so compare readings to service information rather than a universal number.
Common causes of P0171
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Vacuum leak from a cracked hose, intake gasket or intake boot
- Dirty or failing mass air flow (MAF) sensor under-reporting airflow
- Leaking or stuck-open positive crankcase ventilation (PCV) valve or hose
- Weak fuel pump, clogged fuel filter or faulty pressure regulator
- Restricted or dirty fuel injectors
- Exhaust leak ahead of the upstream oxygen sensor
- Leaking brake booster or its vacuum hose
- Unmetered air entering after the MAF sensor, such as a loose dipstick or oil cap
P0171 symptoms
- Check engine light on
- Rough or unstable idle
- Hesitation, stumbling or lack of power when accelerating
- Misfire codes on some engines
- Hard starting in some cases
- Hissing sound from a vacuum leak
How P0171 is diagnosed
- Scan for all codes and review freeze frame data, noting engine load, speed and fuel trims when P0171 set; check for P0174, which would point to a cause shared by both banks.
- Watch short and long term fuel trims at idle and at a steady higher engine speed; trims that improve with engine speed suggest a vacuum leak, while trims that worsen under load suggest a fuel delivery or MAF problem.
- Inspect intake hoses, the intake boot, PCV hoses and vacuum lines; perform a smoke test of the intake system if available.
- Inspect and clean the MAF sensor with a sensor-safe cleaner, and compare its readings with expected values for engine size and speed.
- Test fuel pressure and volume under load to rule out a weak pump or clogged filter.
- Check for exhaust leaks upstream of the bank 1 sensor 1 oxygen sensor.
- Verify the upstream oxygen sensor responds normally so a faulty sensor is not misreporting a lean mixture.
Typical fixes for P0171
- Repair vacuum leaks, replacing cracked hoses, intake gaskets or the intake boot
- Clean or replace the mass air flow sensor
- Replace the PCV valve or its hoses
- Replace the fuel pump, fuel filter or pressure regulator if fuel delivery is low
- Clean or replace restricted fuel injectors
- Repair exhaust leaks ahead of the upstream oxygen sensor
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
P0171 is commonly reported on many Ford, GM, Toyota, Nissan and Volkswagen engines, often traced to aging intake gaskets, PCV hoses or valves, or contaminated MAF sensors. On direct-injection engines, fuel pressure faults can also play a role; definitions can vary by manufacturer; check factory service information.
Related codes & warning lights
P0171 FAQ
What does code P0171 mean?
P0171 means the engine computer has reached the limit of how much fuel it can add on bank 1, because the engine is getting too much air or too little fuel. The standard definition is "System Too Lean (Bank 1)."
Can I drive with a P0171 code?
Usually yes for a short time if the engine runs smoothly, but do not ignore it. A lean mixture can cause hesitation, misfires and higher combustion temperatures, so diagnose it within days, and stop driving if the check engine light starts flashing.
What is the most common cause of P0171?
Vacuum leaks are the most frequent cause, often a cracked hose, a split intake boot or a leaking PCV system. A dirty mass air flow sensor that under-reports incoming air is another very common culprit.
If I have both P0171 and P0174, what does that tell me?
When both banks are lean at the same time, the cause is usually something shared by the whole engine, such as a dirty mass air flow sensor, a large intake leak, a PCV problem or low fuel pressure, rather than a single injector or a leak on one side.
Will cleaning the MAF sensor fix P0171?
Sometimes. If the sensor is dirty, cleaning it can restore accurate airflow readings and bring fuel trims back to normal. If trims stay high afterward, continue looking for vacuum leaks or fuel delivery problems.
Keep researching
Engine guides by brand (known issues, specs)
More fuel & air metering codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.