PO171 and PO174 Diagnostic Guide for 1998 Ford F-150: Causes and Next Steps

Ask your mechanic what causes P0171 and P0174. If they can't explain it, consider finding another technician. These codes on a 1998 Ford F-150 have been widely discussed in this forum—use the search function to find past threads. Both P0171 and P0174 indicate that both engine banks are running lean. When oxygen sensors detect lean exhaust gases, the Powertrain Control Module (PCM) increases fuel injector pulse width to correct the air-fuel ratio. However, if there's a vacuum leak or a Mass Airflow (MAF) sensor issue, the PCM attempts to compensate by adding more fuel—but it can't deliver enough to restore balance, resulting in these codes. While replacing the fuel filter is a good starting point, the first step when diagnosing a Ford with P0171/P0174 should be checking fuel pressure and reviewing fuel trim data using an OBD2 scan tool. A clogged fuel filter might contribute, but Ford vehicles are especially prone to MAF sensor failures—where the sensor underestimates airflow into the engine. Additionally, intake manifold leaks are a common failure point in Fords and can trigger either one or both of these codes. Ensure your mechanic performs a thorough diagnostic rather than making assumptions.

In over 90% of cases, the root cause of P0171 and P0174 on Ford vehicles is a faulty Mass Airflow (MAF) sensor. Always check for vacuum leaks as well.

Please review existing posts before posting duplicate content. This information has already been thoroughly discussed in the forum.

Thread moved to P0XXX category for proper classification.