P0171 on a 2015 GMC Terrain

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 2015 GMC Terrain

Ranked from 315 NHTSA owner complaints filed for this vehicle.

Sort by:

1
PCV valve failure / clogged PCV system
Medium severity
Owners report a PCV valve problem on this engine tied to crankcase pressure and oil-sludge buildup in the manifold; a faulty PCV disrupts the calibrated air path the ECU expects, pushing fuel trim lean.
Reported in 3 NHTSA complaints
Parts:PCV valve
2
Weak fuel pump or clogged fuel filter
Medium severity
One owner's oil-consumption debris was reported clogging the high-pressure fuel pump, restricting delivery and producing a lean condition under load.
Reported in 1 NHTSA complaint
Parts:fuel pump fuel filter
3
Vacuum or intake air leak (hoses, gaskets)
Medium severity
A cracked hose or gasket lets in unmetered air that the MAF never counted, driving bank fuel trims lean; smoke-test the intake.
Common cause
4
Dirty or failing MAF sensor
Medium severity
An under-reporting MAF causes the PCM to under-fuel relative to actual airflow; check MAF grams/sec against expected values.
Common cause
Parts:mass airflow sensor
5
Intake manifold gasket leak
Medium severity
A leaking gasket admits air downstream of the throttle body without being metered by the MAF; look for oil staining or a hiss at idle.
Common cause
6
EVAP purge valve stuck open
Low severity
A purge valve that fails open continuously bleeds extra air/vapor into the intake, skewing fuel trims lean.
Common cause
Parts:EVAP purge valve

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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