Fan Speed Sensor Circuit Range/Performance


The PCM has detected an incorrect amount of voltage (high or low) in the electric cooling fan control circuit.

Code Set Parameters

Voltage variations that vary by greater than 10-percent of the manufacturer’s specified reference voltage will cause a code to be stored and possibly a service engine soon lamp to be illuminated. Some vehicles require multiple drive cycles (usually three) for the service engine soon lamp to be illuminated. If there is a code stored and no service engine soon lamp, the code may be displayed as a pending code.


Symptoms will likely be limited to only service engine soon lamp illumination and a stored code. However, they may also include an overheating engine (when the vehicle is not in motion) and reduced air conditioner efficiency.

Common Causes

Common causes include a bad cooling fan motor or a faulty cooling fan relay. Less common causes include corroded or loose electrical connectors, a faulty engine coolant temperature sensor, and open or shorted electrical wiring (often resulting in a blown fuse).

Common Misdiagnosis

Technicians report the replacement of cooling fan motors without performing a thorough diagnosis results in many unsuccessful repairs. Cooling fan relays have been the subject of several technical service bulletins and recall campaigns in late model vehicles.


  • Most late model vehicles utilize a system of one or more electric cooling fans for engine cooling and air conditioner system operation
  • These fans are categorized as a primary cooling fa and one or more secondary fans
  • Manufacturer’s typically use numeric terminology to differentiate between individual cooling fans and related circuitry
  • The primary fan is usually referred to as the “Fan 1” and the remaining fans are labeled in sequence (“Fan2” and “Fan 3”)
  • Single speed engine cooling fans are the most common but several automakers also use electric fans with multiple speed settings
  • Still other models are equipped with electro-hydraulic engine cooling fans that rely on voltage and hydraulic pressure (usually generated by the power steering pump) to control operation and fan speed. Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
  • Begin with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • All of these engine cooling fan systems are primarily controlled by the PCM, which uses input data received from the engine coolant temperature sensor to calculate fan activation and/or speed
  • When the engine reached a predetermined temperature (225-degrees Fahrenheit is most common), the PCM provides a voltage signal to the fan relay
  • The fan relay is supplied with a constant supply of fused voltage, as well as one or more ground signals and the PCM signal (which may also be a ground signal)
  • The engine cooling fan relay/s help to transform a low voltage (or ground) signal from the PCM into the battery level voltage signal required to operate the cooling fan motor
  • By using a scanner to activate the engine cooling fan, and carefully performing a quick voltage and ground test (battery voltage is normal) at the cooling fan motor, you may successfully determine whether the fan motor is faulty or a lack of voltage/ground is the culprit. If no voltage is present at the primary cooling fan motor, begin by testing the system fuses
  • If all of the fuses are good, locate the engine cooling fan relay and compare voltage readings at the connector to the manufacturer’s specifications for fan operation
  • Replace or repair relays, fuses, or open/shorted circuitry as required
  • If there is no output signal from the PCM, connect a suitable scanner and observe engine temperature
  • If engine temperature readings are not within manufacturer’s specifications, suspect a faulty engine coolant temperature sensor
  • Test the sensor by comparing actual temperature to resistance values with the manufacturer’s specified temperature to resistance values
  • If temperature to resistance values fail to coincide, then replace the engine coolant temperature sensor
  • Most models use one engine coolant temperature sensor for engine drivability and another for temperature gauge operation. If disconnecting the engine coolant temperature sensor connector yields no change in the engine temperature reading displayed on the scanner, suspect shorted or open wiring or electrical connectors in the cooling fan circuit
  • Disconnect the PCM connector prior to checking circuit resistance and compare your findings with manufacturer’s specifications
  • Repair open or shorted circuitry as required and recheck the system
  • PCM failure is rare but possible. Secondary engine cooling fans are a little more complex in operation
  • Secondary engine cooling fans are usually controlled by dual systems
  • Secondary fans located on the engine side of the radiator are often responsible for assisting with cooling the engine, as well as providing air flow for the air conditioning condenser
  • Secondary fans located on the opposite side of the radiator from the engine are usually dedicated to providing air flow for the a/c condenser, only
  • Electric condenser fans are also activated by the PCM, but instead of relying upon an input signal from the engine coolant temperature sensor, it uses a signal from a pressure sensor that is normally located in (or on) a pressurized component of the air conditioning system
  • As the air conditioning compressor begins to operate, and pressure increases in the a/c system, an electrical circuit in the pressure switch is completed
  • This process supplies the PCM with a signal
  • The PCM in turn, sends a voltage (or ground) signal to the fan relay which transfers the low voltage (or ground signal) to a battery voltage signal, which is required for fan operation
  • Cooling fans that are used for a dual purpose (engine cooling and a/c condenser air flow) are activated by either the engine coolant temperature sensor signal and/or the a/c pressure signal (both via the PCM)
  • Test secondary cooling fan operation and circuitry in much the same manner as you would the primary cooling fan
  • Replace components and repair circuitry as needed. Multi-speed cooling fan motors are monitored by the PCM for precise RPM levels
  • Fans that spin too rapidly, or too slowly, will trigger a stored code and a service engine soon lamp
  • These types of systems typically utilize a system of resistors to regulate voltage and vary fan speed
  • Obtaining a fan RPM to voltage conversion chart for your vehicle will be instrumental in determining the cause of a fan overspeed or underspeed condition
  • Compare actual readings to the manufacturer’s specifications for your particular vehicle
  • If voltage readings coincide with manufacturer’s specifications, the fan motor is faulty and if the do not, suspect the resistor assembly or rheostat
  • PCM failure is rare. Electro-hydraulic problems are almost always caused by a low fluid condition
  • Since the fan motor is driven by battery voltage and hydraulic pressure from the power steering pump, power steering leaks should be repaired before attempting to diagnose or repair this condition
  • Power steering pump failure is also a possibility (especially where a low fluid condition has been allowed to prevail)