P1173 — Fuel system too rich

Diagnostic algorithm

1. Connect an OBD-II scan tool, record freeze frame data and current/long-term fuel trims and related codes; clear codes if appropriate then reproduce.
2. Inspect for obvious fuel leaks, strong fuel odor, wet injectors or fuel dripping at rail/injectors.
3. Monitor live data at idle: LTFT/STFT, O2/AFR sensor readings, MAF signal, fuel rail pressure, ECT. Note which values indicate a rich condition.
4. Check fuel pressure with a gauge (static and during cranking/running). Compare to spec; test regulator function (vacuum port if present) and note excessive pressure that can cause overfueling.
5. Test EVAP purge valve operation: command off/on with scan tool and verify it seals at idle; a stuck-open purge can introduce raw fuel vapors and cause rich conditions.
6. Inspect and, if necessary, clean or test the MAF sensor and intake for restrictions or leaks that would affect airflow measurement.
7. Inspect injector electrical connectors and perform a cylinder balance or injector flow test to identify leaking or over‑flowing injector(s).
8. Verify ECT reading with an independent thermometer; replace if sensor is cold-biased.
9. Verify air‑fuel sensor/O2 sensor operation: compare sensor output to known-good behavior (rapid switching for narrowband; expected stoichiometric/wideband values). Replace if sensor is slow, stuck or biased.
10. If wiring or ECM faults are suspected, perform back-probing and resistance checks of sensor circuits and grounds. Repair wiring or connectors as needed.
11. After repairs, clear codes and perform a road test/steady-state checks to confirm fuel trims and AFR/O2 sensors return to normal ranges.