P0494 - Fan Speed Low
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Fan Speed Low (P0494). The signal is below the lower threshold, typically from a short to ground or an internally shorted sensor.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Fan Speed Low – Signal Voltage | Voltage (V) | < 0.20 V (fault state) | Ignition ON, connector engaged |
| 5 V Reference Voltage | Voltage (V) | 4.95 - 5.05 V | Ignition ON at sensor connector |
| Sensor Ground Voltage Drop | Voltage (V) | < 0.05 V (< 50 mV) | Under load to chassis ground |
| Signal Wire Continuity | Resistance (Ω) | < 0.5 Ω (Open = OL) | Disconnected harness between sensor and ECM |
Ignition ON, connector engaged
Ignition ON at sensor connector
Under load to chassis ground
Disconnected harness between sensor and ECM
Oscilloscope Waveform Analysis
100 ms/div, 1 V/divPossible Causes
Short to ground on the signal wire or an internal short circuit within Fan Speed Low
Chafed or pinched wiring harness along the signal path to Fan Speed Low
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Fan Speed Low
Defective internal output driver or reference circuit in the control module for Fan Speed Low
Symptoms
Recommended Solution
Measure signal wire resistance to chassis ground with connector unplugged (must exceed 10 kΩ; low reading confirms short to ground).
Verify loaded supply voltage and harness terminal pins at Fan Speed Low.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Fan Speed Low.
Unplug the sensor:the signal must swing to a known rail. Replace the sensor or repair the wiring based on the result.
Mechanic's Tip
Always run a loaded voltage-drop test on the sensor ground with ignition ON. A drop above 0.1 V shifts the whole signal and tricks the module into setting sensor codes on a good sensor.