P0171 on a 2016 Mazda Cx 5

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 2016 Mazda Cx 5

Ranked from 507 NHTSA owner complaints filed for this vehicle.

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1
Failing in-tank (low-pressure) Denso fuel pump
High severity
Several owners had this pump diagnosed after codes P0192 (low fuel rail pressure) and P0171 (system too lean) appeared together, with symptoms of rough running, hard starting, and stalling — a direct, repeatedly-documented match for this code on this vehicle.
Reported in 4 NHTSA complaints
Parts:fuel pump fuel pump module
2
Failing high-pressure fuel pump
High severity
Separate complaints describe the high-pressure pump itself needing replacement (twice for one owner) with loss of acceleration and stalling, consistent with insufficient fuel delivery producing a lean trim.
Reported in 3 NHTSA complaints
Parts:high-pressure fuel pump
3
Vacuum or intake air leak
Medium severity
Cracked intake boots, PCV hoses, or gaskets let in unmetered air, a standard cause of a lean fuel-trim code.
Common cause
4
Dirty or failing mass airflow (MAF) sensor
Medium severity
A contaminated or drifting MAF sensor under-reports incoming air, causing the ECM to under-fuel and register a lean condition.
Common cause
Parts:mass airflow sensor
5
Clogged fuel filter restricting flow
Medium severity
A restricted filter reduces fuel volume to the rail independent of the pump itself, a standard lean-condition cause.
Common cause
Parts:fuel filter
6
Intake manifold gasket leak
Medium severity
A leaking gasket draws unmetered air past the MAF sensor's measurement point, a standard P0171 cause.
Common cause
Parts:intake manifold gasket
7
PCV valve stuck open
Low severity
A PCV valve that doesn't regulate properly can pull excess unmetered air into the intake at idle.
Common cause
Parts:PCV 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