Where is the cooling fan sensor?

In most systems there isn’t a single universal “cooling fan sensor.” The location depends on the device: cars typically rely on a coolant temperature sensor or a radiator fan switch to drive the fan, while computers use motherboard temperature sensors and fan controllers to regulate speed. This article explains where to look in both contexts and how to identify the right component.

Understanding where this sensor sits is essential whether you’re diagnosing an overheating issue in a car or tweaking fan behavior on a desktop or laptop. Below, you’ll find a detailed guide that separates automotive and computing contexts, plus practical tips to locate and test the relevant components.

Automotive cooling fans: what to look for

Before listing typical locations and sensor types, note that modern vehicles may use one of several arrangements to control the radiator fans. Some rely on a simple coolant temperature switch that completes a circuit to energize a relay, while many newer cars rely on a coolant temperature sensor fed to the engine control unit (ECU), which then commands the fan(s) through a relay or a dedicated fan controller. Dual-fan setups and variable-speed fans add additional complexity but share the same goal: keep engine coolant within an optimal temperature range.

• Coolant Temperature Sensor (CTS): Usually threaded into the engine block, cylinder head, or near the thermostat housing. It sends a digital or analog signal to the ECU or fan controller, which decides when and how fast to run the fans.


• Radiator Fan Temperature Switch (fan switch): A simple thermoswitch installed in or near the radiator or coolant hose. When coolant reaches a threshold, it closes the circuit and activates the fan via a relay. This is common on older vehicles or some budget trims.


• Dual or separate sensors for high/low speed fans: Some systems use two sensors or separate circuits to run a high-speed and a low-speed fan (or two fans) at different thresholds, coordinated by the fan control module.

Concluding the automotive section, locating the sensor often means tracing the wiring from the radiator fan relay to either the thermostat housing region or the engine block, then verifying the sensor’s connector, wiring continuity, and any fault codes with an OBD-II scanner. If a fan won’t engage, start by testing the CTS or the fan switch and inspect the relevant fuses and relays.

Common sensor types in cars

Two main categories cover most vehicles: a coolant temperature sensor that reports to the ECU and a radiator fan switch that directly controls the fan through a relay. The exact placement can vary by make, model, and year, but the CTS is commonly found near the engine block or thermostat housing, while the fan switch is often located in the radiator or near the thermostat housing as a separate component.

Computers and laptops: locating the cooling fan sensor

In computing devices, there isn’t a single sensor attached to the fan itself. Instead, temperature sensors measure the system’s heat at critical points, and the motherboard or a dedicated fan controller uses that data to adjust fan speed. The result is that the “sensor” you care about for controlling a fan is usually a temperature readout from the CPU, GPU, or system chipset, not a physical sensor on the fan.

• Motherboard temperature sensors: Found on the motherboard itself, with common hotspots near the CPU socket, GPU area, and chipset. These sensors provide the data used to govern fan behavior in software and firmware.


• Fan headers and controller circuits: Desktop and laptop boards offer 3-pin (voltage-controlled) or 4-pin (PWM) fan headers. The fan itself often lacks a temperature sensor; instead, it responds to control signals based on sensor data read by the motherboard.


• BIOS/UEFI and software tools: System firmware (BIOS/UEFI) and software packages (HWInfo, SpeedFan, motherboard utilities) read sensor data and allow you to set fan curves or automatic profiles.

In practice, to locate and troubleshoot a cooling fan issue in a computer, you’ll typically check the BIOS/UEFI hardware monitor page for temperatures, inspect fan header connections, and use a software tool to confirm which sensors are feeding the fan controller and what speeds are being reported. If a fan isn’t spinning or is running at the wrong speed, verify fan header compatibility (PWM-capable headers for 4-pin fans), check for dust, and confirm that the software profile is active.

How fan speed is controlled in modern PCs

Many modern systems rely on PWM (pulse-width modulation) to vary fan speed, allowing for smooth, precise adjustments based on temperature readings. The PWM signal is generated by the motherboard or a dedicated fan controller, and the corresponding fan’s 4th wire handles the modulation. If your fan is on a 3-pin header, it’s typically a voltage-controlled setup with speed linked to supply voltage rather than PWM.

Practical steps to locate the sensor in your device

The following steps apply to both automotive and computing contexts, helping you identify the right component and confirm its status before replacing parts or adjusting settings.

1. Identify symptom and context: overheating engine or rising system temperatures, fan not engaging, or abnormal fan noise.


2. Consult the relevant manual or service documentation: wiring diagrams in vehicles or motherboard manuals for PCs/servers often show sensor and fan controller locations.


3. Inspect obvious locations first: CTS near the thermostat housing or engine block in cars; motherboard near CPU socket or chipset for PCs.


4. Check wiring and connectors: look for loose plugs, corroded terminals, bent pins, or damaged insulation on the sensor or fan cables.


5. Test with basic tools: swap a known-good relay or fan, or use a diagnostic tool to read coolant temperature (cars) or system temperatures (PCs/Laptops).


6. Verify software and firmware settings: ensure fan curves are enabled in BIOS/UEFI and software, and that sensors are being read correctly.

Concluding this practical checklist, precise identification of the cooling fan sensor requires attention to the device type and the cooling system design, followed by targeted testing of the sensor, control module, and wiring to confirm where a fault originates.

When to seek professional help

If you’re dealing with persistent overheating, ambiguous sensor readings, or hardware that won’t respond to standard troubleshooting, consider a professional diagnostic. In cars, a technician can safely test the CTS, fan switch, relays, and cooling system pressure. In computers, a technician can verify sensor data integrity, diagnose faulty fans or controllers, and reapply thermal management configurations without risking hardware damage.

Summary

There isn’t a single universal cooling fan sensor. In cars, the sensor or switch is usually a coolant temperature sensor or radiator fan switch located near the engine, thermostat housing, or radiator, with the ECU or fan controller issuing the drive signal. In computers, the “sensor” behind fan control is typically the motherboard’s temperature sensors (CPU, GPU, chipset) read by BIOS/firmware and software tools to regulate fan speed via PWM or voltage control. Locating the exact component involves identifying the device type, checking manuals for sensor and fan-header layouts, inspecting wiring, and using diagnostic tools to confirm readings. Whether on the road or at your desk, a systematic check of sensors, controllers, and connections is the best path to resolution.

In closing, always start with the device’s manual, verify sensor signals with the appropriate diagnostic tools, and ensure the fan control path—from sensor to controller to fan—is functioning correctly. A little testing and proper configuration can restore reliable cooling in both automotive and computing environments.

Where is the cooling sensor located?

The engine coolant temperature (ECT) sensor is typically located near the thermostat housing or in the engine block, often where the upper radiator hose connects to the engine. Some vehicles may have more than one sensor, with one on the engine and another on the radiator to control cooling fans. You can find it by following the upper radiator hose towards the engine or by looking for a component with a wire harness connector attached to it near the thermostat.
 
This video demonstrates how to locate the engine coolant temperature sensor: 59sCarsNToysYouTube · Feb 13, 2018
Common locations

• Thermostat housing: This is the most common location. The sensor is mounted on the housing, which is often near the front of the engine. 
• Engine block: In some cases, the sensor is screwed directly into the engine block. 
• Radiator: Some modern vehicles have a second sensor on the radiator to help control the cooling fan. 

You can watch this video to see how the engine coolant temperature sensor is replaced: 55sRed Pill GarageYouTube · Feb 19, 2022
How to identify it

• Follow the radiator hose: Trace the path of the upper radiator hose from the radiator to where it connects to the engine, often called the thermostat housing or upper intake manifold. The sensor is usually located at this connection point. 
• Look for the electrical connector: The sensor will have a wire harness plugged into it. This connector will have a locking tab that you need to press to detach it. 
• Use your vehicle's documentation: If you are still having trouble, search online using your vehicle's make and model, or check a service manual to see diagrams of your specific engine. 

What happens when you have a bad radiator fan sensor?

A signal from the coolant temperature sensor is what triggers the ECU to turn on the radiator fan, and if it's faulty, it can prevent the fan from working at all.

What sensor controls the cooling fan?

The cooling fan is controlled by the engine coolant temperature sensor (ECT sensor), which signals the engine control unit (ECU) to activate the fan when the engine coolant reaches a specific temperature. In older systems, a separate cooling fan switch performs this function, but most modern vehicles rely on the ECU and the ECT sensor working together. 
This video explains how to test a coolant fan switch: 57sCarsNToysYouTube · Jan 28, 2022

• Engine Coolant Temperature (ECT) Sensor: This is the primary sensor. It measures the temperature of the coolant circulating through the engine. 
• Engine Control Unit (ECU): The ECU receives the temperature information from the ECT sensor. When the temperature rises above a preset threshold (often around 185–200∘185 – 200 raised to the composed with power185–200∘F), the ECU sends an electrical signal to turn on the cooling fan. 
• Cooling Fan Switch (in older vehicles): Some older vehicles have a dedicated fan switch that is physically immersed in the coolant. When the coolant reaches a certain temperature, the switch completes a circuit to activate the fan relay and turn the fan on directly. 

You can watch this video to see how a cooling fan relay works: 1mFixbookYouTube · Feb 10, 2021