What does it mean?
OBD II fault code P0494 is a generic code that is defined as “Engine coolant blower motor speed –low” and is set when the PCM (Powertrain Control Module) detects that the radiator cooling fan(s) is/are not rotating at the desired speed given the current operating conditions and engine coolant temperature. Note that code P0494 refers to electrically operated radiator fans, and not viscous or belt-driven fans.
The purpose of the radiator cooling fan is to draw ambient air through the radiator core to reduce the engine coolant temperature, and as such, it plays a critical role in engine temperature management. Note though that while some applications use a single fan that runs at a constant speed, other applications may use multiple fans. With multiple fan systems, one or more fans may come into operation, depending on the engine coolant temperature, while on single-fan systems.
The fan may operate at different speeds, again depending on the engine coolant temperature. In practice, the radiator fan motor can be activated in several ways, depending on the application.
In some cases, a dedicated engine coolant temperature sensor may activate the fan motor via the PCM, in other cases, and most commonly, a simple bi-metal switch that is in direct contact with the engine coolant closes a circuit that activates the radiator fan motor via a dedicated relay when the coolant reaches a specified temperature. In a few cases, the radiator fan motor is controlled by a series of resistors that both activate and regulate the speed of the fan motor.
Nonetheless, regardless of design specifics, the PCM monitors the fan speed when it is in operation, and will set code P0494 (and may illuminate a warning light) when it detects that the fan’s actual speed deviates from the fan’s desired speed by about 10%. The image below shows a typical radiator fan switch, such as might be found on many modern applications today. Note however that the appearance of fan switches vary greatly between applications.
Refer to the manual for the affected application to determine the operating principles and switching mechanism(s) of the fan motor control circuit.