What does OBD code P0171 mean?
P0171 means the engine management system has detected that the air-fuel mixture on Bank 1 is too lean — there is too much air relative to fuel. This is detected by the oxygen sensors. Common causes include a vacuum leak (unmetered air entering the engine), a dirty mass airflow sensor, or a weak fuel pump. Symptoms include rough idle, hesitation on acceleration, and poor fuel economy.
Likely causes
Vacuum or intake air leak
Cracked hose or loose connection allowing unmetered air in — common after engine work
Dirty or faulty mass airflow (MAF) sensor
MAF measures incoming air — a dirty sensor underreports air, causing lean mixture
Faulty oxygen/lambda sensor
A failed upstream sensor can falsely report lean conditions
Weak fuel pressure
Failing fuel pump or clogged fuel filter — engine cannot deliver enough fuel
Clogged fuel injector
Reduced fuel delivery on affected cylinder(s)
Repairs you may need
Typical cost range: £40–£350 — MAF sensor cleaning: low cost. Lambda sensor: £100–250. Fuel filter: £50–150. Vacuum leak fix: £80–200.
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