P0171 on Subaru
System Too Lean (Bank 1)
P0171 on Subaru vehicles indicates system too lean (bank 1). Au7o has documented this code across 8 Subaru models — most commonly on Ascent, Baja, Crosstrek. This code means the engine's computer has detected that the air-fuel mixture on Bank 1 (the side of the engine with cylinder #1) is running too lean — meaning there is too much air relative to fuel. The computer watches the oxygen sensors and has had to add a large amount of extra fuel (long-term fuel trim) to try to compensate, but it has hit its correction limit. A lean condition can cause rough idle, hesitation, reduced power, and over time can run the engine hotter than normal. It usually points to either unmetered air leaking into the engine or not enough fuel being delivered. Typical repair costs on Subaru range from $1,200 to $10,000, depending on the specific model and root cause.
Common Causes of P0171
- •Vacuum or intake air leak (cracked hose, gasket, or PCV system)
- •Dirty or failing mass airflow (MAF) sensor
- •Weak fuel pump or restricted fuel filter
- •Clogged or dirty fuel injectors
- •Faulty or aged upstream oxygen sensor
- •Leaking intake manifold gasket
- •Low fuel pressure or failing fuel pressure regulator
- •Exhaust leak ahead of the oxygen sensor
P0171 on Subaru by Model
Subaru Ascent(1 issue)
- FA24F PCV Hose Oil/Fuel/Hot-Rubber Odor Under Hood (TSB 11-204-23)2019-2022
Owners report a strong oil, gasoline or hot-rubber smell from the engine bay, sometimes accompanied by oil seepage at the PCV hose joint. Subaru traced this to an undersized PCV hose clamp on the FA24F that allows blow-by vapors and oil mist to escape. TSB 11-204-23 prescribes a revised PCV hose assembly and improved spring clamp.
Subaru Baja(1 issue)
- EJ25 Head Gasket Failure2003-2006
The Subaru Baja's 2.5L EJ25 naturally-aspirated engine suffers from the same chronic head gasket failure that affects all EJ25-equipped Subarus. The composite gaskets degrade over time, typically between 80,000-150,000 miles, causing external oil and coolant leaks. The Baja's engine placement and bed-over-engine design can make leaks harder to spot until they become severe. The issue affects all 2003-2006 NA Bajas. Turbo models use a different gasket design and are less susceptible.
Subaru Crosstrek(1 issue)
- FB20 Engine Excessive Oil Consumption2013-2017
The 2.0L FB20 engine in first-generation Crosstreks (2013-2017) shares the same oil consumption defect as the FB25 used in other Subaru models. Defective piston rings fail to maintain proper oil control, leading to oil burning at rates of 1 quart every 1,500-2,500 miles. Subaru covered this under warranty extension and the Oakes v. Subaru class-action settlement. The issue is most prevalent in 2013-2015 models.
Subaru Forester(1 issue)
- FB25 Engine Excessive Oil Consumption2011-2018
The 2.5L FB25 engine in 2011-2018 Foresters is widely reported for excessive oil consumption, often burning 1 quart every 1,000-1,500 miles. The root cause is defective piston rings that fail to maintain proper oil control. Subaru acknowledged the issue with TSB 02-157-14R and extended the powertrain warranty to 8 years/100,000 miles for affected vehicles. A class-action settlement (Oakes v. Subaru) covered ring replacement for qualifying owners. The oil consumption test (OCT) required by Subaru involves monitoring oil usage over 3,000 miles at the dealership.
Subaru Impreza(1 issue)
- EJ253 Head Gasket Failure (External Leak)1999-2011
The 2.5L EJ253 naturally-aspirated engine in 1999-2011 Imprezas is notorious for external head gasket failure. The composite head gaskets degrade over time, allowing coolant and oil to seep externally at the gasket mating surfaces. Unlike a typical blown head gasket that causes coolant-oil mixing, Subaru EJ25 head gaskets typically leak externally first. The boxer engine design means gaskets are at the bottom of the heads, making leaks drip downward and often go unnoticed. If left unaddressed, external leaks can progress to internal coolant-oil mixing and overheating.
Subaru Legacy(2 issues)
- EJ25 Head Gasket Failure2000-2009
The 2.5L EJ25 engine in 2000-2009 Legacys suffers from the same head gasket failure that plagues all EJ25-equipped Subarus. The composite gaskets degrade, causing external oil and coolant leaks at the head-to-block mating surfaces. The Legacy's higher-mileage highway use means many owners encounter this between 80,000-150,000 miles. Phase 1 engines (2000-2005, single overhead cam) are more prone to coolant leaks, while Phase 2 engines (2006-2009, DOHC) tend to leak oil externally first.
- FB25 Engine Oil Consumption2010-2016
The 2010-2016 Legacy 2.5i uses the FB25 engine which is affected by the same oil consumption issue as other FB25-equipped Subarus. Defective piston rings allow excessive oil to bypass into the combustion chamber. The Legacy's higher average speeds mean oil consumption can be even more noticeable during highway driving. Subaru's oil consumption warranty extension and the Oakes v. Subaru settlement cover affected vehicles.
Subaru Outback(3 issues)
- Head Gasket Failure1999-2010
The 2.5L non-turbo engine is known for external head gasket leaks. Oil seeps past the gasket and can contaminate the coolant. Subaru extended warranties and improved gasket design in later years.
- Excessive Oil Consumption2011-2019
The FB25 2.5L engine is known for excessive oil consumption, sometimes using 1 quart every 1,000-2,000 miles. Subaru extended warranty coverage and offered piston ring replacement for affected vehicles. Most common on 2011-2014 models.
- Head Gasket Issues2011-2019
While the FB25 has improved head gasket reliability compared to older EJ engines, some vehicles still experience head gasket failures. External oil leaks and coolant leaks can develop, particularly at higher mileage.
Subaru WRX(2 issues)
- Excessive Oil Consumption2015-2021
The FA20DIT engine can consume oil at a rate higher than expected, sometimes up to 1 quart every 1,000-3,000 miles. This is often related to the turbocharger, PCV system, or piston ring design. Subaru considers up to 1 quart per 1,200 miles as within normal range.
- Piston Ringland Failure2015-2021
The FA20DIT engine can suffer from piston ringland failure, especially when tuned or running high boost. The ringland (area between piston rings) can crack, causing compression loss and engine damage. This is often related to detonation from improper tuning or low octane fuel.
Looking for P0171 on a different make?
View P0171 across all makes →Frequently Asked Questions
What does P0171 mean on Subaru?▼
P0171 stands for "System Too Lean (Bank 1)." This code means the engine's computer has detected that the air-fuel mixture on Bank 1 (the side of the engine with cylinder #1) is running too lean — meaning there is too much air relative to fuel. The computer watches the oxygen sensors and has had to add a large amount of extra fuel (long-term fuel trim) to try to compensate, but it has hit its correction limit. A lean condition can cause rough idle, hesitation, reduced power, and over time can run the engine hotter than normal. It usually points to either unmetered air leaking into the engine or not enough fuel being delivered. On Subaru specifically, this code is documented across 8 models.
What causes P0171 on Subaru vehicles?▼
Common causes on Subaru: Vacuum or intake air leak (cracked hose, gasket, or PCV system), Dirty or failing mass airflow (MAF) sensor, Weak fuel pump or restricted fuel filter, Clogged or dirty fuel injectors, Faulty or aged upstream oxygen sensor. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0171 on a Subaru?▼
Repair costs on Subaru range from $1,200 to $10,000, depending on the specific model and root cause.
Which Subaru models have P0171 documented?▼
Au7o has documented P0171 on 8 Subaru models: Ascent, Baja, Crosstrek, Forester, Impreza, Legacy, Outback, WRX.