* Reduced Power/Limp Mode: If the onboard diagnostic system (OBD-II) detects a serious engine fault (e.g., a sensor malfunction, low oil pressure, severe misfire), the engine's power output will be significantly reduced. This isn't a specific "fail-safe mode," but a symptom of various underlying problems. The truck will still run, but likely at a very reduced power level, usually with a check engine light illuminated.
* Individual Cylinder Deactivation (if equipped): Some 2005 F-150s had cylinder deactivation technology. If a cylinder malfunctions, the system might deactivate it to prevent damage, reducing engine power. This isn't a "fail-safe mode" in itself but a reactive measure.
* Transmission limitations: Problems detected by the engine might also cause the transmission to shift into a safe mode, limiting speeds and potentially preventing shifting to certain gears.
* Check Engine Light Illumination: The check engine light is the primary indicator of a problem that *could* lead to a reduced power situation. The code stored by the OBD-II system will help diagnose the specific issue.
In short: There's no single "Engine Fail-Safe Mode" name in the 2005 F-150's documentation. Instead, various systems react to problems by reducing power, limiting functionality, and illuminating warning lights. The best way to understand what's happening is to check the illuminated warning lights (check engine light is the most common) and get the diagnostic trouble codes (DTCs) read using an OBD-II scanner. These codes will pinpoint the specific fault causing the reduced performance.
