Common Checks
- Inspect the MAF sensor for contamination.
- Check for leaking injectors, high fuel pressure, or a restricted air filter.
- Review oxygen-sensor data for a biased upstream sensor.
Diagnostic trouble code
P0172 System Too Rich, Bank 1
The PCM is subtracting too much fuel on bank 1 because the mixture is rich.
Fuel Trim & Mixture CodesCommon Repairs
- Clean or replace a contaminated MAF sensor.
- Repair fuel-pressure or injector faults.
- Replace a biased oxygen sensor only after fuel and air checks pass.
Mustang Notes
- Rich codes can appear after intake or tune changes that alter expected MAF scaling.
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.