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.
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.
Related codes
Often seen with P0171
Read & clear P0171 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX