Dirty or faulty mass air flow (MAF) sensor (over-oiled air filters can contaminate it). Vacuum leak after the MAF sensor. Cracked vacuum or PCV hose or connection. Faulty or stuck-open PCV valve. Control module software that needs updating (RepairPal). Check for TSBs or a dealer reflash. Failed upstream oxygen sensor (Bank 1 Sensor 1). Sticking, plugged, dirty or failed fuel injector. Low fuel pressure: a plugged fuel filter or weak fuel pump. Exhaust leak between the engine and the first O2 sensor.

Clean or replace the MAF sensor. Repair vacuum, PCV or intake leaks. Replace a faulty PCV valve or O2 sensor. Restore fuel pressure (filter, pump or regulator). Repair or replace faulty injectors.

P0171⛽ Fuel / air meteringDrive with caution, fix soon

P0171: System Too Lean (Bank 1) (Meaning, Causes, Fixes)

Q: What does P0171 mean?

P0171 means the oxygen sensor on Bank 1 sees too much oxygen in the exhaust, and the engine computer can't add enough fuel to hold the ideal 14.7:1 air-fuel ratio. A dirty MAF sensor or a vacuum leak is often the cause. The upstream (front) oxygen sensor on Bank 1 reads the air-fuel mixture leaving the cylinders. The PCM adjusts fuel to keep the ratio near 14.7:1. When it can't, because there's too much air, it sets P0171. On V6/V8/V10 engines, Bank 1 is the side with cylinder #1. P0171 is very similar to P0174 (Bank 2), and the two often show up together.

Q: Can I keep driving with P0171?

Drive with caution, fix soon. Usually drivable short-term; you may not notice anything. But a lean engine can hesitate, knock or run rough, and lean conditions are a common cause of misfires. Get it fixed soon rather than ignoring it.

Reviewed 2026-10-21 · Based on 2 cited source(s)

What P0171 means

The upstream (front) oxygen sensor on Bank 1 reads the air-fuel mixture leaving the cylinders. The PCM adjusts fuel to keep the ratio near 14.7:1. When it can't, because there's too much air, it sets P0171. On V6/V8/V10 engines, Bank 1 is the side with cylinder #1. P0171 is very similar to P0174 (Bank 2), and the two often show up together.

Severity: can I keep driving?

Drive with caution, fix soon

Usually drivable short-term; you may not notice anything. But a lean engine can hesitate, knock or run rough, and lean conditions are a common cause of misfires. Get it fixed soon rather than ignoring it.

Common symptoms

Check engine light on (often the only symptom)
Lack of power
Spark knock (detonation)
Rough idle
Hesitation or surge on acceleration
'Coughing' or misfiring, and trouble idling when warm or at a stoplight (RepairPal)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Dirty or faulty mass air flow (MAF) sensor (over-oiled air filters can contaminate it)
Vacuum leak after the MAF sensor
Cracked vacuum or PCV hose or connection
Faulty or stuck-open PCV valve
Control module software that needs updating (RepairPal). Check for TSBs or a dealer reflash.
Failed upstream oxygen sensor (Bank 1 Sensor 1)
Sticking, plugged, dirty or failed fuel injector
Low fuel pressure: a plugged fuel filter or weak fuel pump
Exhaust leak between the engine and the first O2 sensor

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Clean the MAF sensor with a cleaner that's safe for it. Be careful not to damage it, and let it dry before reinstalling.
Inspect all vacuum and PCV hoses, and every hose and connection in the intake, for cracks or loose fittings.
Check the intake manifold gaskets for leaks.
If you have a scanner with live data, watch short- and long-term fuel trims.
Check for a dirty fuel filter and, with a gauge, the fuel pressure.
Ask a dealer, or search NHTSA, for technical service bulletins. RepairPal lists outdated control-module software as a common cause.

Typical fixes

Clean or replace the MAF sensor
Repair vacuum, PCV or intake leaks
Replace a faulty PCV valve or O2 sensor
Restore fuel pressure (filter, pump or regulator)
Repair or replace faulty injectors

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

You can't find the leak: a shop can run a smoke test
Fuel pressure testing needs a gauge and the right adapter
The code returns after the cheap fixes

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

What fixes P0171 most often?

OBD-Codes.com says cleaning the MAF sensor and fixing vacuum leaks often fixes it, though not always.

Why do I have P0171 and P0174 together?

Both banks are lean, which points to something they share, such as a MAF problem, a vacuum leak, or low fuel pressure.

Is P0171 urgent?

Sources differ. RepairPal rates it 'Urgent'; OBD-Codes.com says you more than likely won't notice drivability problems. Both agree the oxygen sensor is a common misdiagnosis, so diagnose before replacing it. Note: lean means too much air (oxygen), not enough fuel.

Should I replace the O2 sensor?

RepairPal lists oxygen sensors as the common misdiagnosis for P0171. Check for vacuum leaks, the MAF and fuel delivery first.

Sources

Last reviewed 2026-10-21. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.