OBD codes / P0171
P0171
SeriousSystem Too Lean (Bank 1)
P0171 is usually safe to drive with short-term, but do not ignore it. A lean mixture runs hotter and can eventually trigger misfires or, in the worst case, contribute to engine damage. Have it diagnosed soon — and because a cheap vacuum leak and an expensive fuel pump both set this code, confirm the cause before buying parts.
What this code means
Your engine constantly adjusts the air-to-fuel ratio. "Too lean" means there is too much air or too little fuel, and the computer has had to add fuel to compensate up to its limit on Bank 1 (the side with cylinder 1). The most common cause is unmetered air sneaking in — a vacuum or intake leak, or a dirty mass-airflow (MAF) sensor that under-reports airflow. It can also be weak fuel delivery: a tired fuel pump, a clogged filter, or dirty injectors. A persistent lean condition can raise combustion temperatures and, over time, lead to misfires, so it is worth chasing down.
Symptoms you might notice
- Rough or unstable idle
- Hesitation, stumble, or lack of power under acceleration
- Slightly reduced fuel economy
- Occasional surging at steady speed
- Sometimes no symptom beyond the check-engine light
What it costs to fix
Typical range: $100–$1,000 · about ~0.3–3 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Vacuum / intake leak repair (hose, gasket, PCV) | $10–$150 | ~0.5–2 hrs |
| MAF sensor (clean or replace) | $0–$350 | ~0.3–0.5 hrs |
| Fuel pump | $150–$500 | ~1–3 hrs |
| Fuel filter | $20–$120 | ~0.5–1.5 hrs |
A vacuum-leak or MAF-clean fix is often under $200; a fuel pump is the expensive end. The MAF low end can be near-zero if cleaning the sensor solves it. Diagnosis time to find a small leak can itself add labor.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0171 for my exact vehicle →
How to fix P0171
Moderate — doable at home with common tools and patience
- Read the fuel trims with a scan tool
With the engine warm, watch long-term fuel trim (LTFT) on Bank 1 at idle and again at 2,500 RPM. If the trim is high at idle but drops closer to normal at higher RPM, that points strongly to a vacuum leak; if it stays high at all speeds, suspect the MAF or fuel delivery instead.
- Hunt for vacuum leaks
With the engine idling, listen for hissing and spray small bursts of carb cleaner (or use a smoke machine) around the intake manifold gasket, vacuum hoses, PCV hose, and intake boot. If the idle changes when you hit a spot, you've found unmetered air getting in — repair or replace that hose or gasket.
- Clean the MAF sensor
Remove the mass-airflow sensor and clean the sensing element with MAF-specific cleaner only (never carb cleaner or touching the wires). A coated element under-reports airflow and makes the computer under-fuel; if trims improve after cleaning, you're done.
- Check fuel pressure and delivery
Connect a fuel-pressure gauge (or read fuel pressure on the scan tool) and compare against spec at idle and under load. Low pressure means a tired pump, clogged filter, or weak regulator is starving the engine of fuel.
- Clear the code and verify
After the repair, clear the code and drive through a mix of idle, cruise, and acceleration while watching that LTFT settles into single digits. If trims stay normal over a few drive cycles, the lean condition is fixed.
DIY or shop? Most of this is DIY-friendly: reading trims, spraying for vacuum leaks, and cleaning the MAF need only basic tools and a cheap scanner. A proper smoke test and fuel-pump replacement (often inside the tank) are where a shop earns its money — smoke machines find leaks carb cleaner misses, and dropping a fuel tank is a big job. Start with the cheap checks; many P0171s are solved by a cracked hose or dirty MAF.
Tools for the DIY steps: OBD2 scanner with live data · Carb cleaner or a smoke machine · MAF sensor cleaner · Fuel pressure gauge · Flashlight and basic hand tools
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0171.
P0171 on specific vehicles
The likely causes shift by model. We’ve written up the patterns we see most:
Frequently asked
Can I drive with P0171?
Short-term, usually yes — it is rarely an immediate breakdown risk. But a lean condition runs the engine hot and can lead to misfires over time, so have it diagnosed soon rather than letting it linger.
What is the most common cause of P0171?
Unmetered air is the usual culprit: a vacuum or intake leak, or a dirty MAF sensor reading low. Weak fuel delivery (pump, filter, injectors) is the next thing to check. A smoke test plus a fuel-pressure check usually settles which it is.
Will cleaning the MAF sensor fix P0171?
Sometimes — a dirty MAF is a common, cheap cause, and sensor cleaner is inexpensive. But if a vacuum leak or fuel-delivery problem is the real issue, cleaning the MAF won't help, so verify before assuming.
How much does it cost to fix P0171?
Often inexpensive — many fixes are a vacuum hose, a gasket, or a MAF clean for well under $200. It only gets costly if the cause is a fuel pump. Confirm the cause first so you don't replace parts that were fine.
Can P0171 cause a misfire?
Yes. A persistently lean mixture can lead to misfires (and codes like P0300) because some cylinders aren't getting enough fuel. Fixing the lean condition often clears the related misfire too.
Seeing P0171 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0171 for my vehicleSymptoms that trigger P0171
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0171 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.