Common Checks
- Check for vacuum leaks, cracked PCV elbows, intake tube splits, and loose clamps.
- Inspect the MAF sensor and air filter assembly.
- Check fuel pressure and injector operation if no air leak is found.
Diagnostic trouble code
P0171 System Too Lean, Bank 1
The PCM is adding too much fuel on bank 1 because the mixture is lean.
Fuel Trim & Mixture CodesCommon Repairs
- Repair vacuum leaks or split PCV/intake hoses.
- Clean or replace a contaminated MAF sensor.
- Address weak fuel delivery only after air metering checks pass.
Mustang Notes
- On modular V8 Mustangs, aged PCV hoses and intake boots are frequent lean-code starting points.
Model-Year Applicability
Listed for 1996 and newer traditional gasoline Mustang pages in this first-pass catalog. Engine, calibration, and Ford-specific codes may narrow that range as more service data is added.