P0171 on a 2017 GMC Acadia Denali

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 2017 GMC Acadia Denali

Ranked from 639 NHTSA owner complaints filed for this vehicle.

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1
Mass airflow/manifold pressure (MAF/MAP) sensor fault
Medium severity
One owner had the MAP sensor replaced for a recurring check-engine light tied to the "intake air pressure measurement system," with the engine shuddering and stalling at idle - an inaccurate air reading here directly produces an incorrect (lean) fuel trim calculation.
Reported in 1 NHTSA complaint
Parts:MAP sensor MAF sensor
2
EVAP purge valve not fully closing (unmetered air path)
Medium severity
GM issued special coverage for an EVAP purge valve on this platform that may not fully close; a purge valve that won't seat acts like a vacuum leak, pulling unmetered air into the intake when it should be shut, which can pull fuel trim lean.
Reported in 1 NHTSA complaint
Parts:purge valve
3
Vacuum or intake leak
Medium severity
A cracked hose, loose intake boot, or leaking gasket lets unmetered air past the MAF sensor, directly producing a lean fuel trim code.
Common cause
4
Weak fuel pump or clogged fuel filter
Medium severity
Insufficient fuel delivery under load can't keep pace with commanded fuel, producing a lean condition that worsens with RPM/load.
Common cause
Parts:fuel pump fuel filter
5
Stuck-open PCV valve
Low severity
A PCV valve stuck open draws excess unmetered air into the intake, leaning the mixture, especially noticeable at idle.
Common cause
Parts:PCV valve
6
Exhaust leak upstream of the upstream O2 sensor
Low severity
Outside air drawn in ahead of the front oxygen sensor is read as excess oxygen, causing the PCM to lean the mixture further than it should.
Common cause
7
Intake manifold gasket leak
Low severity
A failed intake gasket is a common unmetered-air path producing a steady lean condition.
Common cause
Parts:intake manifold gasket

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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