P0171 on a 2013 Subaru Forester

P0171 means the engine computer is adding more fuel than normal on bank 1 because it sees too much air or not enough fuel.

Not your vehicle? See general P0171 info.

Most common causes on the 2013 Subaru Forester

Ranked from 77 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Excessive Oil Consumption / Low Engine Oil
High severity
The engine burns oil at an abnormally high rate, leading to low oil levels which can cause internal engine damage, stalling, or trigger check engine lights. Confirm by checking the dipstick and performing an oil consumption test.
Reported in 12 NHTSA complaints
2
Catastrophic Engine Failure / Short Block Failure
High severity
Often a secondary result of oil starvation or rod bearing failure, leading to knocking, stalling, and complete engine failure. Confirmed by metal in the oil, loud knocking, and engine lockup.
Reported in 6 NHTSA complaints
3
Blown Head Gasket
High severity
Coolant and oil mixing or external leaks can cause overheating, poor running conditions, and trigger codes. Confirmed by fluid cross-contamination (oil in coolant/water in oil) or a block tester.
Reported in 4 NHTSA complaints
4
Valve Cover and Camshaft Seal Leaks
Medium severity
External oil leaks from the valve cover and camshaft seals can cause low oil levels and vacuum leaks if the PCV system is affected. Confirmed by visual inspection of oil leaking from the engine.
Reported in 1 NHTSA complaint
5
Faulty Oil Pressure Switch/Sensor
Low severity
A failing oil pressure switch can cause the oil light to flicker or stay on even when oil levels are full. Confirmed by testing oil pressure manually and checking the sensor for leaks or electrical faults.
Reported in 1 NHTSA complaint
6
Vacuum Leak (Intake Manifold Gaskets, Vacuum Hoses, PCV)
Medium severity
Unmetered air enters the engine, causing a lean condition (P0171). Confirmed by a smoke test or listening for hissing noises around the intake.
Common cause
7
Dirty or Faulty Mass Air Flow (MAF) Sensor
Low severity
A dirty MAF sensor misreports the amount of air entering the engine, causing the ECM to under-fuel the engine. Confirmed by checking MAF g/s readings on a scan tool.
Common cause
8
Weak Fuel Pump or Clogged Fuel Filter
Medium severity
Low fuel pressure prevents the injectors from delivering enough fuel, resulting in a lean mixture. Confirmed by testing fuel pressure with a mechanical gauge.
Common cause
9
Faulty Upstream Oxygen (O2) Sensor or Air/Fuel Ratio Sensor
Medium severity
A degraded O2 sensor can falsely report a lean condition or fail to read the mixture correctly. Confirmed by monitoring sensor voltage/current data on a scan tool.
Common cause

These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0171 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Rough idle
Hesitation
Surging idle
Poor fuel economy

What to check before replacing parts

Review short- and long-term fuel trims on bank 1

Look for: High positive trims (commonly worse at idle, improving with RPM) mean the computer is adding fuel to compensate for excess air or insufficient fuel — that pattern is this code's fingerprint.

Smoke-test the intake

Look for: Smoke escaping (or a hiss you can hear) at intake boots, gaskets, or vacuum hose connections confirms unmetered air is entering after the MAF sensor, which is the most common cause of a lean code.

Inspect the PCV hoses and valve

Look for: A cracked, split, or disconnected PCV hose, or a PCV valve stuck open, draws in extra unmetered air the same way a vacuum leak does.

Clean and test the MAF sensor

Look for: Visible dirt or oil film on the sensor element, or a grams/second reading that doesn't track expected airflow for RPM, means the MAF is under-reporting air and causing the computer to under-fuel.

Verify fuel pressure

Look for: Pressure below spec (weak pump, clogged filter, failing regulator) starves the engine of fuel and produces the same lean trim pattern as a vacuum leak — rule it out before chasing leaks.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Car Runs Rough