P0171 — System Too Lean (Bank 1)
P0171 means the engine is running lean: too much air, not enough fuel. It is usually driveable, but a lean mixture runs hot and can damage the converter and pistons if it is ignored.
The engine will usually run, though it may idle roughly or hesitate. A sustained lean mixture raises combustion temperature and damages expensive parts over time. This code will cause an emissions test failure.
What P0171 means
The engine control module adjusts fuel delivery continuously using feedback from the upstream oxygen sensor. When it has to add more than roughly 25 percent extra fuel to reach the correct mixture and still cannot get there, it concludes the fault is outside its authority and sets P0171. The cause is almost always unmetered air entering after the mass airflow sensor, or a fuel system that cannot keep up.
| System | Powertrain — fuel and air metering |
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Symptoms you may notice
What causes P0171
Unmetered air entering after the airflow sensor
A cracked intake boot, a perished vacuum hose, or a failed intake manifold gasket lets air in that the sensor never measured. This is the most common cause by a wide margin.
Contaminated mass airflow sensor
Oil residue from an over-oiled performance air filter coats the sensing element, so the sensor under-reports airflow and the module injects too little fuel.
Common misdiagnosis
Clean the sensor with dedicated MAF cleaner before replacing it; solvent damage is a common self-inflicted fault.
Fuel pump or filter cannot maintain pressure
A tiring pump or a clogged filter holds pressure at idle but falls short under load, so the mixture goes lean exactly when the engine needs fuel most.
Exhaust leak ahead of the upstream sensor
A leak before the front oxygen sensor draws in ambient air between exhaust pulses, so the sensor reads lean and the module adds fuel it did not need to add.
How to diagnose it
Read short and long term fuel trim at idle and at cruise
With the engine warm, record both fuel trim values at idle and again at a steady 2500 rpm. The pattern tells you where to look before you touch a single part.
Inspect the intake tract for leaks
Examine the airflow sensor boot, every vacuum hose, the throttle body gasket, and the brake booster hose for cracks and loose clamps. Flex the intake boot with the engine idling and listen for a change.
Inspect and clean the mass airflow sensor
Remove the sensor and inspect the sensing wire for oily film or debris. Clean it only with dedicated mass airflow cleaner and let it dry completely before refitting.
Measure fuel pressure under load
Fit a fuel pressure gauge and compare readings at idle and under a road load against the manufacturer specification. A pump that holds pressure at idle can still fall short under demand.
Safety
Fuel lines are under pressure and petrol vapour ignites easily. Depressurise the system and work away from any ignition source.
Related codes
Common questions
Is it safe to drive with a P0171?
For short trips usually yes, but a lean mixture raises combustion temperature. Left alone it can damage the catalytic converter and, in the worst case, the pistons.
Why did P0171 come back after I cleared it?
Clearing the code erases the record, not the fault. The module re-runs its fuel trim monitor within a drive cycle or two and sets the code again if the lean condition is still present.
Last verified 2026-03-12 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.