P0171
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
The car usually still drives, but sustained lean running raises combustion temperature, can cause misfire and detonation, and over time damages spark plugs, exhaust valves and the catalytic converter. A large vacuum leak can also cause stalling in traffic.
- Safe to drive
- Short trips are usually fine if it idles and accelerates cleanly. Stop driving if it misfires, stalls, or pings under load.
System Too Lean (Bank 1)
The engine computer has been adding extra fuel on bank 1 for a long time and still cannot get the mixture where it wants it. Something is letting in air the computer never measured, or the fuel system is not delivering what it should.
In closed loop the PCM adjusts injector pulse width from oxygen sensor feedback. Short-term fuel trim reacts within seconds; long-term fuel trim is the learned correction stored per load and RPM region. P0171 sets when the correction sits near the positive limit for a sustained period — commonly somewhere around plus 20 to plus 25%, though the exact threshold is calibration-specific. Bank 1 is the bank containing cylinder 1; on an inline engine that is the entire engine.
How the car detected itThis is a continuous monitor running whenever the engine is in closed loop. The PCM watches learned long-term trim in specific load and RPM cells and sets the code when correction stays past the calibrated limit, usually after failing on two consecutive trips. Because the correction is learned per load region, the freeze frame RPM and load tell you where the fault lives: lean only at idle points to an unmetered air leak of fixed size, which matters less as airflow rises; lean under load points to fuel delivery volume or an airflow sensor under-reporting.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Short trips are usually fine if it idles and accelerates cleanly. Stop driving if it misfires, stalls, or pings under load.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- Expect worsening idle quality, hard cold starts, and eventually misfire codes. Sustained lean running overheats exhaust valves and the catalytic converter — a $10 cracked vacuum hose left alone can turn into a converter bill. Fuel economy typically gets worse, not better, because the computer keeps commanding more fuel.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Vacuum leak in intake or PCV system | Very common | — |
| 2 | MAF sensor contaminated, under-reading | Very common | — |
| 3 | Fuel pressure below specification | Common | — |
| 4 | Fuel injectors partially clogged | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Freeze frame RPM and engine load
It tells you whether the engine ran lean at idle, at cruise, or under load. Lean at idle but normal under load means a vacuum leak. Normal at idle but lean under load means fuel supply volume or airflow measurement.
Visible and audible intake leaks — cracked intake boot, split PCV or brake booster hose, loose oil filler cap or dipstick, disconnected vacuum line
This is the most common cause and it costs nothing to check. A hiss that changes pitch when you pinch a hose finds it in minutes. Treat a split brake booster hose as urgent in its own right: it also weakens power brake assist, so the pedal goes hard and stopping distance grows.
The mass air flow sensor element
A contaminated hot wire or film under-reports airflow, so the PCM injects too little fuel. Oil film from an over-oiled aftermarket air filter is a classic cause.
Whether P0174 is also set on a two-bank engine
Both banks lean means a shared cause — MAF, fuel pressure, or a leak at the throttle body or plenum. One bank only points at that bank's injectors, intake gasket, or oxygen sensor.
The intake ducting between the MAF sensor and the throttle body
Any leak downstream of the MAF is air the computer never counted. Cracks hide on the underside of the accordion boot and only open when the engine rocks under load.
Read live fuel trims across the whole operating range
Tools Scan tool with live data and graphing
With the engine fully warm and in closed loop, record short-term and long-term trim at idle, at 1500 to 2000 RPM with no load, and under load in gear. The shape of the trim across conditions is the single most informative test on this code.
ExpectedHealthy combined trim sits within roughly plus or minus 10%. Trim high at idle that falls toward zero as RPM rises indicates a fixed-size air leak. Trim near normal at idle that climbs under load indicates fuel volume or a MAF under-reporting.
Smoke test the intake system
Tools Smoke machine, intake and exhaust plugs
Plug the intake and exhaust openings, introduce low-pressure smoke into the intake tract, and inspect gaskets, hoses, injector seals, throttle body, dipstick tube, oil filler and the PCV path. This replaces guessing and finds leaks that only open when hot or under vibration. Include the brake booster and its check valve — a leaking booster diaphragm leans the engine and costs braking assist at the same time.
ExpectedNo smoke escaping anywhere in the intake tract, the brake booster path, or the crankcase ventilation path.
Rule out an exhaust leak ahead of the upstream oxygen sensor
Tools Exhaust smoke machine or stethoscope
Do this while the smoke machine is already connected, before any fuel-system work. A leak at or before the upstream sensor draws in outside air on exhaust pulses. The sensor honestly reports lean, the PCM adds fuel it does not need, and you get high fuel trim from a fuel system that is working perfectly. Skipping this is what turns a manifold gasket into a replaced fuel pump.
ExpectedNo leak at the manifold, manifold gasket, flex pipe, or sensor bung.
Test whether the MAF sensor is reporting plausible airflow
Tools Scan tool with live data, MAF-specific cleaner
Compare calculated engine load at wide-open throttle near peak torque RPM against what a healthy engine should produce. A MAF that under-reports shows up as calculated load well below what the throttle position and RPM demand. Cleaning the element with a dedicated MAF cleaner and rechecking is a valid confirmation step.
ExpectedCalculated load approaching 90 to 100% at wide-open throttle on a healthy naturally aspirated engine. Sensor-specific gram-per-second values are vehicle-specific and should be compared against service data, not memory.
Inspect the crankcase ventilation system
Tools Visual inspection, smoke machine
A PCV valve stuck open, a torn PCV hose, or a failed oil separator diaphragm feeds unmetered air straight into the plenum. Inspect the valve for free rattle, check the hoses for softening and splits, and include the whole path in the smoke test.
ExpectedPCV valve seals in the closed direction, hoses intact and pliable, no smoke escaping from the crankcase ventilation path.
Check fuel pressure and, importantly, fuel volume under load
Tools Fuel pressure gauge with the correct adapters, or scan tool fuel pressure PID on returnless and direct-injection systems
Relieve fuel system pressure before opening any fuel connection, keep ignition sources away, and have an extinguisher within reach — a fuel rail holds pressure long after the engine is shut off. Measure static pressure at key-on, running pressure at idle, and pressure held during a hard snap throttle or a loaded test. Pressure that is correct at idle but sags under demand points to a weak pump, a restricted filter, or a clogged in-tank strainer. Pressure alone is not enough — a pump can hold pressure and still not flow. On direct-injection engines do not open the high-pressure side: read the pressure PIDs with a scan tool instead.
ExpectedPressure holds its specification under load with only a small drop. The specification is vehicle-specific; returnless systems typically run considerably higher than older return-style systems.
Test injector delivery
Tools Stethoscope, scan tool with injector control, or bench flow tester
If air metering and fuel supply are clean, test the injectors. Options in increasing order of certainty: listen with a stethoscope for even operation, run a scan-tool injector balance test, or remove and bench flow-test. Before removing anything, relieve fuel pressure and disconnect the battery, and fit new O-rings on reassembly. On direct-injection engines this involves high-pressure fuel lines and belongs in a workshop.
ExpectedAll injectors delivering within a few percent of each other.
Confirm the repair by forcing a trim relearn
Tools Scan tool
Clear the learned fuel trims, then drive through idle, cruise and load conditions. Recheck trims. Only after they settle should you run a drive cycle to confirm the monitor passes.
ExpectedCombined fuel trim settles within roughly plus or minus 10% across the operating range and stays there.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace cracked intake boot, vacuum hose, or PCV hose | Smoke escaping at that point, or fuel trim dropping when the leak is temporarily sealed | $50–$250 | DIY |
| Replace a leaking brake booster hose, check valve, or booster diaphragm | Smoke escaping at the booster or its hose, or a hard brake pedal with poor vacuum assist alongside the lean condition | $80–$700 | DIY |
| Replace intake manifold or throttle body gasket | Smoke escaping at the gasket line, or trims changing when propane or smoke is directed at the joint | $200–$700 | Hard |
| Clean or replace the mass air flow sensor | Calculated load implausibly low at wide-open throttle, or trims correcting after cleaning and staying corrected through a relearn | $50–$500 | DIY |
| Replace PCV valve | Valve stuck open or failing to seal in the closed direction | $60–$250 | DIY |
| Replace crankcase oil separator / CCV assembly | Smoke escaping from the separator body, or its diaphragm failing to hold vacuum. Access often requires intake manifold removal, which is most of the cost | $250–$900 | Hard |
| Replace fuel pump or fuel filter | Fuel pressure or volume below specification, particularly pressure collapsing under load | $250–$1,400 | Hard |
| Clean or replace fuel injectors | Injector balance or bench flow test showing one or more injectors delivering low | $150–$1,200 | Hard |
| Repair exhaust leak ahead of the upstream oxygen sensor | Leak located at the manifold, gasket, flex pipe or sensor bung with the exhaust pressurized | $150–$800 | Hard |
The spread on this code is real: a split vacuum hose is a $50 fix, while an in-tank fuel pump on a vehicle where the tank has to come down can pass $1,200. This is a code where paying for diagnosis pays off — an hour of smoke testing and fuel-trim reading typically costs $100 to $200 and narrows the possible repairs down to one.
- Replacing the upstream oxygen sensor. It is usually reporting the truth — the engine really is running lean.
- Cleaning the MAF sensor and declaring it fixed without verifying the reading afterwards or clearing the learned trims.
- Ignoring the second bank on a V engine. P0171 alone and P0171 with P0174 point to very different places.
- Spraying carburetor cleaner or starting fluid around the intake to hunt leaks. It works, but it is a fire hazard on a hot engine and degrades rubber. A smoke machine is the right tool.
- Chasing injectors and fuel pumps before checking for an exhaust leak upstream of the oxygen sensor.
- Adding a bottle of injector cleaner, clearing the code, and calling that a diagnosis.
- Mass Air Flow (MAF) Sensor
- Fuel Pump
- Fuel Injector
- PCV Valve
- Intake Manifold
- Fuel Pressure Regulator
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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