A laptop fan that does not spin is a serious problem: the laptop overheats, throttles and eventually shuts down. Some laptops refuse to boot at all and show a fan error. The cause can be the fan itself, its supply, the signals that control it, the embedded controller that decides how fast it should run, or the temperature readings the EC relies on. This guide walks through each, in the order that finds the fault quickest.
How a laptop fan is controlled
| Wire or signal | Typical name | Job |
|---|---|---|
| Supply | FAN_5V, +5V_FAN or similar | Powers the motor |
| Ground | GND | Return |
| Speed control | FAN_PWM or similar | A pulse signal setting the speed (4-wire fans) |
| Speed feedback | FAN_TACH, FAN_SPEED or similar | Pulses per revolution, read by the EC |
Most modern laptop fans have four wires. The EC sets the speed by changing the PWM duty cycle and checks the speed through the tach signal. Some older or simpler fans have three wires and are controlled by varying their supply voltage through a fan driver or MOSFET instead. The fan connector is described under fan header, and duty cycle under duty cycle.
The EC decides the speed from temperature readings: the processor's internal sensor, thermistors on the board, and sometimes sensors near the battery or regulators.
Step 1: test the fan by itself
Disconnect the fan from the board and power it directly:
- Set a bench supply to 5 V with a low current limit.
- Connect the fan's supply and ground wires.
- On a four-wire fan, the PWM wire can usually be left unconnected — many fans run at full speed without a PWM signal. Check the fan's behaviour rather than assuming.
If the fan does not spin, or the supply hits its current limit, the fan is faulty: seized bearing, broken winding, damaged internal driver. Replace it. If it spins freely, the fan is fine and the problem is on the board.
Also check mechanically: blades touching the housing, a fan blocked by dust, or a bent heatsink shroud can stop a fan that is electrically fine.
Step 2: the fan supply
With the fan connected and the laptop on, measure the supply pin at the connector.
- About 5 V — supply fine; look at the control signals.
- Zero — trace it back. Fan supplies usually pass through a fuse or a small switch near the connector.
The fan fuse
A blown fan fuse is a common cause, often after a fan with a shorted motor or a pinched cable. Check it with continuity, as in How to Test a Fuse on a Laptop Board with a Multimeter. Before replacing it, measure the fan's supply pin to ground with the fan disconnected and then connected — a short that appears only with the fan connected means the fan itself is shorted.
Switched fan supplies
On some boards the EC switches the fan supply with a MOSFET or a fan driver chip that varies the voltage. If the supply is present before the switch but not after, check the switch's control signal.
Step 3: the PWM signal
With the laptop on and the fan connected:
- Measure the PWM pin with a meter in DC mode. A pulse signal reads as an average voltage roughly proportional to its duty cycle.
- Load the processor or let the laptop warm up — the reading should change as the EC changes the fan speed.
A reading stuck at zero may mean the EC is commanding the fan off, the PWM line is shorted, or a series resistor is open. On many fans, a PWM line held low means "minimum speed" or "stop", so a broken PWM line can leave the fan stopped even though its supply is fine.
Step 4: the tach signal
The tach signal is an open-collector output from the fan, pulled up on the board. With the fan stopped and the laptop on, the tach pin should sit at its pull-up voltage, or at low, depending on the fan's position. With the fan spinning, a meter shows an average between the two levels.
If the tach pull-up is missing or the line is broken, the EC cannot see the fan turning. Many laptops then treat the fan as failed: they show a fan error at power-on, refuse to boot, run the fan at full speed, or shut down. Check the pull-up resistor and the line to the EC.
Step 5: EC control and firmware
If supply, PWM and tach are all healthy and the fan still does not run when it should, look at the decision-maker:
- EC firmware — a corrupted EC firmware can leave fan control in an odd state. On boards where the EC firmware lives in the BIOS chip, a correct BIOS update or reflash may restore it.
- BIOS fan settings — some laptops have fan profiles or a "silent" mode that keeps the fan off at low temperatures. That is normal behaviour, not a fault.
- EC damage — rare, but liquid near the EC can damage its fan pins.
Step 6: temperature readings
The EC only spins the fan when it believes the laptop is warm. If it reads the temperature wrongly, the fan may never start — or may run at full speed constantly.
- Check the temperatures reported in the BIOS or by software.
- If one reading is far from reality, test the corresponding thermistor and its divider, as described in How to Test a Thermistor with a Multimeter.
A fan that runs constantly at full speed while the laptop is cool is the opposite symptom, and the same checks apply. A laptop that is hot at idle even with a working fan is covered in Laptop Running Hot at Idle: Motherboard Causes.
Example: fan error on every boot
A laptop shows a fan error at power-on and turns off after a few seconds. The fan spins normally on a bench supply. On the board, the fan supply is present and the PWM signal changes during start-up, but the tach line sits at zero with the fan connected and disconnected alike. With power removed, the tach line reads normally to ground, but its pull-up resistor is open — cracked, perhaps when the fan connector was forced. A new resistor restores the tach signal, the EC sees the fan turning, and the error is gone.
Quick reference
| Finding | Likely cause |
|---|---|
| Fan does not spin on a bench supply | Fan faulty or blocked |
| No supply at connector | Fan fuse, switch, upstream rail |
| Supply present, PWM stuck | EC command, PWM line, series resistor |
| Fan spins but EC reports an error | Tach line or pull-up |
| Everything fine, fan off when it should run | EC firmware, BIOS fan profile, temperature reading |
FAQ
Can I run a laptop fan directly from 5 V to test it?
Yes, from a current-limited supply. Most four-wire fans run at full speed without a PWM signal.
Why does my laptop refuse to boot because of the fan?
Many laptops check the fan's tach signal at power-on. If they see no rotation, they stop to protect the processor.
Why does the fan run at full speed all the time?
The EC may be reading an excessive temperature, missing the tach signal or running a failsafe after a firmware problem.
Is a noisy fan a board fault?
Usually not. Noise comes from worn bearings or dust. Clean or replace the fan.
Can a shorted fan damage the board?
It can blow the fan fuse or overload the fan supply. Fuses exist for exactly this reason.
Bottom line
Test the fan on a bench supply first. Then check the fan supply and its fuse, watch the PWM signal change with temperature, confirm the tach pull-up, and only then look at EC firmware and temperature sensors. The fan itself, the fuse and the tach line account for most cases. The fan is managed by the embedded controller, so keep its other symptoms in mind.