P0171 (system too lean, bank 1) and P0174 (system too lean, bank 2) mean the engine computer had to add a lot of extra fuel to keep the mixture correct — and it hit the limit of how much it could add. In other words, either the engine is getting unmeasured air, or it is not getting enough fuel. The long-term fuel-trim number that sets these codes is usually above roughly +25%. This guide explains what causes it, how to find the specific fault, and what the repair typically costs.
One bank or both? Start there
The single most useful clue is whether one bank or both banks are lean. It cuts the suspect list in half before you touch a tool.
Both banks lean (P0171 and P0174 together) points to something the whole engine shares: a dirty or failing mass airflow (MAF) sensor under-reporting air, low fuel pressure (weak pump, clogged filter, failing regulator), a large vacuum leak at the intake plenum or PCV, or a restricted air filter's opposite — unmetered air.
One bank lean only points to something local to that cylinder head: a bank-specific intake-manifold gasket leak, a leaking injector on that bank, or an exhaust leak ahead of that bank's oxygen sensor that fools it into reading lean.
The most common causes, ranked
Vacuum / intake air leak — the number-one cause. Cracked intake boots, a torn PCV hose, a leaking brake booster, or a failed intake gasket let in air the MAF never counted. Classic signature: high positive fuel trims at idle that shrink at higher RPM. Confirm with a vacuum-leak test.
Dirty or failing MAF sensor — under-reports incoming air, so the PCM injects too little fuel. Trims that are lean across the board (idle and cruise) point here more than a vacuum leak. Test the MAF before replacing it.
Low fuel pressure / delivery — a tired fuel pump, clogged filter, or leaking pressure regulator starves every cylinder. Lean under load especially.
Exhaust leak upstream of the O2 sensor — a leak before the sensor pulls in outside air, the sensor reads lean, and the PCM adds fuel it didn't need — a false lean code. Worth checking early because it is easy to overlook and mimics a fuel fault.
Clogged or weak injectors — deposits reduce flow; the PCM commands more fuel but the injector can't deliver.
How to diagnose it in order
Read the freeze-frame data — note the RPM and load when the code set. Lean at idle behaves differently from lean at cruise.
Compare fuel trims at idle vs. 2,500 RPM. Trims that are high at idle and drop off at RPM = vacuum leak. Trims high everywhere = MAF or fuel supply.
Do a smoke or vacuum-leak test on the intake.
Check MAF readings against spec (grams/sec at idle and snap-throttle).
Check fuel pressure with a gauge, keyed on and under load.
Watch for an upstream exhaust leak if only one bank is affected.
A persistent lean condition can also cause a misfire and rough idle, so if you also have misfire codes, fix the lean cause first — the misfire often clears with it.
Repair cost (2026 US ranges)
Ranges assume a US independent shop at about $110–$160/hour plus typical parts. "DIY parts" is parts-only if you do the work. Planning figures, not a quote.
Repair
Shop (parts + labor)
DIY parts
Vacuum / intake leak repair
$150–$500
$20–$150
PCV valve / hose
$80–$250
$15–$90
MAF sensor
$200–$450
$80–$300
Fuel pump
$600–$1,200
$150–$450
Fuel pressure regulator
$200–$500
$40–$200
Fuel injector (one)
$300–$700
$60–$300
Upstream exhaust leak repair
$150–$600
$20–$200
Diagnosis
$100–$200
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FAQ
Can I drive with a P0171 or P0174 code?
Short-term, yes, but a lean mixture runs hotter and can damage pistons and the catalytic converter over time, and often causes misfires. Fix it within days, and stop driving if the check engine light starts flashing.
Is a lean code always a vacuum leak?
No. A vacuum leak is the most common cause, but a dirty MAF sensor, low fuel pressure, or an exhaust leak near the O2 sensor produce the same code. Compare fuel trims at idle vs. RPM to tell them apart before buying parts.
How much does it cost to fix a lean code?
From about $80 for a PCV valve or a vacuum hose to $600–$1,200 for a fuel pump. Most lean codes are a cheap vacuum-leak or MAF fix — the key is diagnosing correctly so you don't replace a good pump.
General diagnostic information for US-market vehicles, not a substitute for a hands-on inspection. Costs are 2026 ballpark ranges for planning only. Always follow the correct procedure for your specific vehicle.