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Check Price P0171
System Too Lean (Bank 1)
🟡 SERVICE SOON POWERTRAIN · Fuel & Air Metering
O2 sensor on Bank 1 reports the air-fuel mixture is running leaner than the PCM's correction range allows.
What You'll Notice
- Rough idle or engine stumble at stoplight
- Hesitation on acceleration, especially cold
- Fuel trims LTFT Bank1 above +10%
- MIL (Check Engine Light) illuminated
- Possible surge on highway cruise
Probable Causes
- Vacuum leak — cracked intake manifold gasket, broken PCV hose, loose throttle body boot
- Dirty or failing MAF sensor (most common on GM, Ford V8s)
- Weak fuel pressure: failing fuel pump or clogged fuel filter
- Clogged or stuck-open EGR valve leaking exhaust into intake
- Failing upstream O2 / wideband lambda sensor skewing corrections
Direct Repair Steps
- Smoke-test the intake from throttle body to intercooler — locate and seal any vacuum leak
- Clean MAF sensor with CRC MAF Cleaner spray; replace if LTFT normalizes
- Check fuel pressure at rail; spec is typically 58–60 psi (GM) or 30–40 psi (port injection)
- Inspect and clean EGR passages; replace valve if stuck
- Replace upstream O2 sensor if freeze-frame shows abnormal B1S1 voltage pattern
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Check PriceQuick answers
What does P0171 mean?
O2 sensor on Bank 1 reports the air-fuel mixture is running leaner than the PCM's correction range allows.
Is P0171 serious?
It depends on the system and symptoms, but the page ranks the severity and likely causes so you can decide whether to stop driving or continue with caution.
What should I buy before troubleshooting P0171?
A reliable OBD2 scanner helps confirm the code, read live data, and avoid guessing before repairs.