P0171

System Too Lean Bank 1

Fuel & air metering Generic OBD-II High

What it means

P0171 means the engine control module has detected too much air relative to fuel on bank 1 — the mixture is running lean and fuel trims have hit their correction limit. It is usually caused by an unmetered air leak or a MAF or fuel-delivery problem, and left unfixed it can trigger misfires.

High

Can I keep driving? Usually drivable, but a persistent lean mixture can cause hesitation, rough idle and, if left unfixed, misfires that overheat the catalytic converter — repair it soon.

Real-world scan data

Seen in

8,695

scans in the past year

Found in

1.4%

of scans

2.5%

of scans that found faults

Appears alongside · share of P0171 scans

P0300in 19.1% of scans

P0420in 17.3% of scans

P0130in 16.9% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on
Rough idle, hesitation, or occasional stalling
Reduced power and slower acceleration
Slightly worse fuel economy or a lean surge at cruise

Common causes

Most likely first

Vacuum or intake leak letting in unmetered air (most common)Likely
Dirty or failing mass air flow (MAF) sensor reading lowCommon
Low fuel pressure — weak fuel pump or clogged fuel filterCommon
Clogged or dirty fuel injectorsLess often
Exhaust leak near the upstream O2 sensor skewing readingsLess often
Failing upstream oxygen sensorLess often

How to diagnose & fix

Work through it in order

Read freeze-frame and long/short fuel trims to confirm a true lean condition, then smoke-test the intake for vacuum leaks.
Inspect intake boots, PCV and vacuum hoses, and the intake gasket for cracks or loose connections.
Clean or test the MAF sensor and check for an unmetered air path after it.
Check fuel pressure against spec; service the fuel filter and clean injectors if pressure or delivery is low.
Inspect the exhaust manifold and upstream O2 sensor area for leaks, then clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0171?+

For a short time, yes, but a lean mixture can cause misfires that damage the catalytic converter, so have it fixed promptly.

Can I just clear P0171?+

You can clear it, but it returns within a drive cycle or two unless the vacuum leak, MAF or fuel-delivery cause is actually repaired.