How to Test a Coolant Temperature Sensor Causing the Cooling Fan to Stay on

When your radiator fan keeps running long after you've shut off the engine or never stops even while driving the coolant temperature sensor (CTS) is one of the first things you should check. This small, inexpensive sensor tells your car's computer how hot the engine is. If it sends a false "overheating" signal, the computer responds by keeping the cooling fan on full blast. Testing it yourself can save you a mechanic's diagnostic fee and help you fix the problem in under an hour.

What Does the Coolant Temperature Sensor Actually Do?

The coolant temperature sensor is a thermistor a resistor that changes its electrical resistance based on temperature. It's usually threaded into the engine block or cylinder head, with its tip submerged in coolant. As coolant temperature rises, resistance drops. The engine control module (ECM) reads this resistance as a voltage signal and uses it to control fuel injection, ignition timing, and critically cooling fan operation.

When the ECM detects a temperature reading above a certain threshold (often around 200–230°F depending on the vehicle), it commands the cooling fan relay to close and turn the fan on. If the sensor is faulty and constantly reports a high temperature, the fan never gets the signal to shut off.

Why Would a Bad CTS Make the Fan Stay On All the Time?

There are a few failure modes that cause this specific symptom:

Shorted sensor Internal short causes extremely low resistance, which the ECM interprets as very high temperature.
Open circuit A broken internal element sends infinite resistance. Many ECMs default to full fan-on as a safety measure when they see this.
Stuck high reading The sensor drifts out of spec over time and reads 20–40°F hotter than actual coolant temperature.

In any of these cases, the ECM believes the engine is overheating and runs the fan continuously to protect the engine. If you've noticed your radiator fan won't stop running and suspect the CTS, testing it directly is the fastest way to confirm.

What Tools Do You Need to Test the Coolant Temperature Sensor?

Digital multimeter capable of reading resistance (ohms) and DC voltage
OBD2 scan tool (even a basic one works) to read live coolant temperature data and stored trouble codes
Reference resistance chart for your specific vehicle (found in the factory service manual or online forums)
Infrared thermometer (optional but helpful for cross-checking actual coolant temperature)

How Do You Test the Coolant Temperature Sensor With a Multimeter?

Step 1: Check for Trouble Codes First

Connect your OBD2 scanner and check for diagnostic trouble codes. Codes like P0115, P0116, P0117, or P0118 point directly to the engine coolant temperature circuit. A P0117 means low input (possible short), while P0118 means high input (possible open circuit). These codes don't guarantee the sensor is bad wiring issues can set the same codes but they narrow things down fast.

Step 2: Read Live Data

With the engine cold (sitting overnight is ideal), turn the ignition to "on" without starting the engine. Use your scan tool to read the coolant temperature PID. It should be close to ambient air temperature. If it reads something like 250°F on a cold morning, the sensor or its circuit has a problem.

Step 3: Perform a Resistance Test (Ohms Test)

Locate the CTS. It's typically near the thermostat housing or on the engine block. Your vehicle's service manual will show the exact location.
Unplug the electrical connector from the sensor.
Set your multimeter to the ohms (Ω) setting.
Touch the multimeter probes to the two sensor terminals (for a two-wire sensor).
Record the resistance reading and compare it to the manufacturer's specification at the current temperature.

A typical reading at 68°F (20°C) is somewhere between 2,000 and 3,000 ohms, but this varies by vehicle. At 200°F (93°C), many sensors should read between 200 and 500 ohms. If your reading is near zero (shorted) or shows infinite resistance (OL/open), the sensor is bad.

Step 4: Perform a Voltage Test on the Wiring Harness

If the resistance test shows the sensor itself is within spec, the problem might be in the wiring. Reconnect the sensor, back-probe the connector, and check for voltage:

Turn the ignition on (engine off).
You should see approximately 5V reference voltage from the ECM on one wire and a varying voltage signal on the other.
If there's no 5V reference, inspect the wiring for damage, corrosion, or a break between the ECM and the sensor.
Check the ground wire for continuity to the engine ground.

Step 5: Use a Heat Gun or Hot Water to Verify Sensor Response

For a more thorough test, remove the sensor and place the tip in a container of water. Heat the water with a heat gun or stove while monitoring resistance with your multimeter. The reading should change smoothly and consistently as temperature rises. If it jumps around, flatlines, or doesn't change at all, the sensor is faulty even if it passed a static ohms test.

What Are Common Mistakes When Testing a Coolant Temperature Sensor?

Testing only the sensor and ignoring the wiring A corroded connector, chafed wire, or poor ground can mimic a bad sensor. Always inspect the wiring harness and connector pins for green corrosion or pushed-back pins.
Confusing the CTS with the temperature gauge sender Many vehicles have two separate sensors: one for the ECM (CTS) and one for the dashboard gauge. They're not interchangeable. Make sure you're testing the right one.
Not comparing readings to the correct spec Resistance values vary widely between vehicle makes. A reading of 800 ohms might be perfect on one car and completely wrong on another. Always look up the exact specification.
Ignoring the thermostat A stuck-open thermostat can cause erratic temperature readings and confuse both the CTS and the ECM. If temperatures seem inconsistent, check the thermostat as part of your diagnosis.
Replacing the sensor without clearing codes After replacing a bad CTS, always clear the trouble codes with your scan tool. Some vehicles keep the fan running in "limp mode" until codes are cleared.

Could Something Else Be Causing the Fan to Stay On?

Yes. Before you replace the sensor, rule out these other possibilities:

Fan relay stuck closed The relay itself can weld itself in the "on" position. Swap it with an identical relay in the fuse box to test.
Fan control module failure Some vehicles use a separate module to control fan speed. If it fails, the fan may default to high speed.
AC system request If the air conditioning is on, many vehicles command the cooling fan to run regardless of coolant temperature. Make sure the AC is off during testing.
Actual overheating Don't assume the sensor is wrong until you verify coolant temperature independently with an infrared thermometer pointed at the thermostat housing or upper radiator hose.

How Do You Replace a Bad Coolant Temperature Sensor?

If testing confirms the sensor is faulty, replacement is straightforward on most vehicles:

Let the engine cool completely.
Disconnect the negative battery terminal.
Unplug the sensor connector.
Use a deep socket (commonly 19mm or 22mm) to unscrew the sensor. Some coolant will spill have a drain pan ready.
Apply thread sealant (if specified) to the new sensor and thread it in by hand to avoid cross-threading.
Torque to specification, reconnect the connector, and refill any lost coolant.
Clear codes and run the engine to operating temperature while monitoring the fan and scan tool data.

Quick Diagnostic Checklist

✅ Scan for codes (P0115–P0118 point to CTS circuit)
✅ Read live coolant temperature on cold engine should match ambient
✅ Unplug sensor and measure resistance compare to factory spec
✅ Inspect connector and wiring for corrosion, damage, or loose pins
✅ Verify actual coolant temperature with an infrared thermometer
✅ Rule out a stuck-closed fan relay or failed fan control module
✅ If sensor is bad, replace it, refill coolant, and clear codes
✅ After repair, monitor the fan through a full warm-up cycle to confirm the fix

Tip: If you've replaced the sensor and the fan still stays on, the next step is to test the fan relay and inspect the wiring between the CTS and the ECM. A wiring short to ground on the signal wire can trick the ECM into reading high temperature even with a brand-new sensor installed.

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