P0171 means system too lean bank 1: the engine is getting more air than the fuel system can match, and fuel trim has hit its limit. The most common causes are a vacuum leak and a dirty mass airflow sensor, and they are also the cheapest to fix: a cracked vacuum or PCV hose is an inexpensive part, so most of a shop bill is finding the leak, usually with a smoke test, and a dirty MAF sensor can often be cleaned rather than replaced. If fuel delivery is short instead, typical US shop prices are $60 - $180 for a fuel filter (on cars where it is a separate part) and $100 - $300 for a fuel-system cleaning service for dirty injectors; a weak fuel pump is the expensive end. Clean the MAF and check for vacuum leaks before buying parts.
Also written as PO171 or P171.
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P0171 means the engine is running lean on bank 1 - there is too much air relative to fuel, and the computer has run out of room to add fuel to compensate. It is most often caused by unmetered air sneaking into the engine (a vacuum leak) or by the mass airflow sensor under-reporting airflow. Left alone, a persistent lean condition can cause misfires and, over time, engine damage.
Yes for a short trip, though it is worth fixing soon rather than ignoring. A lean condition mostly causes rough idle, hesitation, and mildly reduced power rather than a breakdown, so driving to a shop is normally fine. Left unaddressed for weeks or months, a lean mixture runs hotter than designed and can eventually cause misfires or engine wear, so get it diagnosed within a couple of weeks.
Listed roughly most-likely first. Confirm with a scan tool and the freeze-frame data before replacing parts.
| Fix | DIY parts | At a shop |
|---|---|---|
| Find and seal a vacuum leak | $10 - $50 | $100 - $300 |
| Clean or replace the MAF sensor | $10 - $150 | $50 - $250 |
| Replace the PCV valve or hose | $10 - $40 | $80 - $180 |
| Replace a weak fuel pump | $80 - $300 | $300 - $600 |
Ranges are rounded and for the US market only - actual price depends on region, shop labor rates, and the exact make and model.
What a new MAF sensor costs, if testing points to it
These open an Amazon search - add your year, make and model, because most of these parts are fitment-specific.
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P0171 means bank 1 is running lean - the PCM has been adding fuel to compensate and has hit the limit of what it is allowed to add. Crucially, this code is almost never a fuel problem. It is usually an air problem: the engine is getting more air than the sensors measured, so the fuel the PCM calculated is not enough. Unmetered air entering after the mass airflow sensor is the dominant cause, which is why the diagnosis is largely a hunt for vacuum leaks rather than anything to do with the fuel system.
The name pushes people toward the fuel system, and replacing a fuel pump or injectors for P0171 is an expensive and usually wrong move. Lean almost always means extra unmetered air, not missing fuel. Eliminate every air path before spending anything on fuel components.
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Difficulty: Easy to Moderate. Time: 0.5 - 3 hours. DIY parts: $15 - $250. Shop total: $120 - $600.
One of the better DIY codes, because the most likely causes are a cracked intake tube, a dirty MAF sensor, and a split PCV hose - all cheap, all reachable from the top of the engine, all fixable with hand tools. The only reason to involve a shop is if you need a smoke test to locate a leak you cannot find by hand.
Replace intake hoses, PCV components, and the air filter on schedule. Every common cause of this code is a rubber or plastic part that hardens with heat and age. They fail on a predictable timeline, and replacing them proactively is far cheaper than the misfires and converter damage a long-running lean condition causes. Track the air filter and PCV service in ServiceSage and you replace the parts that cause this code before they crack, rather than after a warning light. Track this maintenance free.
Typical US independent-shop repair cost for P0171 runs about $100 - $600, depending on the root cause. Common fixes: Find and seal a vacuum leak (about $100 - $300 at a shop); Clean or replace the MAF sensor (about $50 - $250 at a shop); Replace the PCV valve or hose (about $80 - $180 at a shop); Replace a weak fuel pump (about $300 - $600 at a shop).
No, not on its own. P0171 will keep coming back every time the relevant monitor re-runs until the underlying cause is actually repaired - clearing it with a scan tool without fixing what triggered it only delays the light coming back on.
It is the full name of P0171. Lean means there is too much air for the amount of fuel. The computer constantly trims fuel up or down to hold the mixture correct; when it has added as much fuel as it is allowed to on bank 1 and the mixture is still lean, it sets P0171. Bank 1 is the side of the engine with cylinder 1 - on a four-cylinder engine, it is the whole engine.
Rarely on its own. A clogged filter reduces airflow, but the mass airflow sensor measures the air that actually gets through, so the mixture usually stays correct. What does cause lean codes is a dirty or oil-contaminated MAF sensor under-reporting airflow - over-oiled reusable filters are a known cause - or unmetered air entering after the sensor through a cracked intake boot or vacuum hose.
P0171 is lean on bank 1 and P0174 is lean on bank 2, which only exists on V6, V8 and other engines with two cylinder banks. When both set together, look for a cause shared by both banks - a dirty MAF sensor, low fuel pressure, or a large vacuum leak at the intake or PCV system. P0171 alone on a V engine points more toward a leak or injector on that one bank.
Related trouble codes: P0174 - System Too Lean, Bank 2, P0101 - Mass Air Flow (MAF) Sensor Circuit Range/Performance, P0172 - System Too Rich, Bank 1, P0300 - Random / Multiple Cylinder Misfire Detected, P0113 - Intake Air Temperature (IAT) Sensor 1 Circuit High
Ford: On Ford V8 trucks and SUVs, cracked PCV and intake vacuum hoses and a contaminated MAF sensor are the documented first checks. When P0171 and P0174 set together, the cause is almost always shared between banks rather than a single injector.
Chevrolet and GMC: On GM V8s, intake manifold gasket leaks are a common source of unmetered air. A smoke test at the intake finds it without guesswork.
BMW: BMWs are known for lean codes from the crankcase ventilation valve and its hoses, which crack with heat. A failing crankcase vent often whistles at idle and makes the oil-filler cap hard to pull off with the engine running.