What are the symptoms of P0171?

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

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

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

Vehicles Affected

chevy gmc ford toyota honda bmw hyundai