HomeTips & GuidesHow To Test a Single-Wire Coolant Temperature Sensor: Signs of a Faulty Unit and Testing Steps

How To Test a Single-Wire Coolant Temperature Sensor: Signs of a Faulty Unit and Testing Steps

In the article below, we’ll show you how to test a single-wire coolant temperature sensor.

Internal combustion engines must run at a specific optimal temperature range to maximize efficiency.

The engine coolant temperature sensor (ECT) is responsible for detecting the motor’s temperature and sending data to the Engine Control Unit for ignition timing and fuel injection adjustment. 

In some cars, this sensor reading can also trigger the engine cooling fan to turn on. So, this device is critical to the engine. Let’s look at its fault/failure symptoms and how to test it below.

Table of Contents

What Is a Single-Wire Coolant Temperature Sensor?

Most vehicles have more than one ECT. The primary coolant temperature sensor sits near the cylinder head thermostat (engine block) or on the thermostat housing, while the second can be on any other engine part but is usually in the radiator.

Both have a similar internal operating mechanism that uses an NTC (Negative Temperature Coefficient) thermistor.

 As the temperature of the coolant increases, the thermistor’s internal resistance decreases, sending the respective voltage signal to the ECU.

To get the most accurate readings, the ECT probe or tip dips into the cooling system passages to immerse into the flowing coolant.

An ECT with the metal probe facing the front

A one-wire coolant temperature sensor operates using this temperature detection mechanism but has a single-wire connection to the ECU.

Therefore, the one-wire sensor receives power and sends/receives data signals on the same bus. 

This communication is digital in some devices (with ethernet wires), where each sensor has a unique ROM ID number.

Benefits of a Single-Wire Coolant Temperature Sensor

  • Enables multiple sensor connections on a single controller/module
  • Requires fewer wires
  • You can connect the sensors in series on a single wire

Drawbacks of a Single-Wire Coolant Temperature Sensor

  • If the series network connection goes down, all sensors go offline
  • Susceptible to communication issues

Signs of a Faulty Coolant Temperature Sensor

The ECU detects voltage signals from the coolant temp sensor to adjust fuel injection, variable valve timing, transmission shifting, radiator fans, ignition timing, and the dashboard temperature gauge. 

Therefore, if it is faulty, you’ll notice these signs.

Engine Overheating

A faulty temperature sensor can send an incorrect cold signal voltage to the ECU, making it deactivate or pause the engine cooling features.

A vehicle’s temperature gauge on the dashboard

A vehicle’s temperature gauge on the dashboard

Additionally, the module will adjust the ignition timing, fuel injection, and variable valve timing to suit cold engine conditions for faster warming. But since the engine is already warm, it will overheat and damage sensitive parts.

Black Smoke from the Exhaust

Coolant temp sensors help the ECU determine the fuel-air mixture to send to the combustion chambers. 

Malfunctioning units sending faulty cold readings will make the engine run rich to raise the engine temperature, making the exhaust emit black smoke.

This emission is bad for the environment and your pocket due to the poor fuel economy.

Difficulty Starting

A faulty coolant temperature sensor can also send incorrect high-temperature readings to the ECU, making it inject a lean fuel-air mixture into the engine.

If cold-starting the vehicle, such faulty readings will make it difficult for the engine to run because the little fuel won’t be enough to burn and turn on the engine in low temperatures.

Radiator Fan Issues

If the coolant sensor sends erroneous data to the ECU indicating high engine temperatures, the computer will turn on the radiator fans. This situation isn’t as bad because the engine will still run.

A mechanic removing the radiator fan assembly from the engine bay

A mechanic removing the radiator fan assembly from the engine bay

But the opposite is worse. Sending wrong low-temperature readings to the ECU when the engine is hot will make it burn and damage sensitive parts.

Transmission Shifting Problems

The Transmission Control Module (TCM) also uses the ECT readings to adjust shifting between normal and overdrive. 

When the engine is cold, the TCM shifts to overdrive.

But it returns to normal mode when the motor is hot to avoid overworking and raising the temperature further. 

Faulty sensors cause inaccuracies in this shifting conversion.

Rough Idling

If a faulty ECT sensor sends fluctuating data to the ECU, it will make the computer continuously adjust the fuel-air mixing between lean and rich. 

This continued fluctuation will run the engine roughly.

Fluctuating Temperature Gauge

The same fluctuations that cause rough idling will make the temperature gauge transition from hot to cold and back multiple times, indicating erratic readings on the dash temp gauge.

Illuminated Check Engine Warning Light

The ECU or PCM constantly monitors the engine coolant sensor to make the necessary adjustments.

A vehicle’s check engine light next to the coolant temperature gauge

How To Test a Single-Wire Coolant Temperature Sensor: A vehicle’s check engine light next to the coolant temperature gauge

If this sensor data is missing or the readings are outside the preset range, the computer will turn on the instrument panel’s check engine warning light and store the trouble code in memory.

You can access the trouble code and its meaning using an OBD II scanner.

How To Test a Single-Wire Coolant Temperature Sensor

Although replacing this sensor is a simple task, you should consider testing it to determine if the issue runs deeper. 

You’ll need a multimeter, cold water, and hot water for this test.

Although it has a single-wire connection, the sensor ground is in its body. 

It connects to the engine block or cylinder head, which connects to the vehicle chassis.

Use these steps to do the test.

Step 1: Locate the Sensor

Turn off the engine and let it cool before undoing parts of the cooling system. 

Find the single-wire coolant temperature sensor in the engine bay near the thermostat.

How To Test a Single-Wire Coolant Temperature Sensor: The coolant temperature sensor location in an old distributor-ignition engine

The coolant temperature sensor location in an old distributor-ignition engine

Follow the upper radiator hose towards the engine. 

The thermostat housing is on the hose’s end, and the engine coolant temperature sensor is right next to it in the form of a tiny black device linked to a wire harness.

Step 2: Unplug the Sensor From the Vehicle

Unplug the sensor using one hand to hold the wiring harness and the other hand to pull the sensor body. Be careful not to snap the wires or force the sensor out.

Step 3: Connect the Sensor to Your Multimeter

Connect the sensor’s body to a metal plate for grounding and attach it to the multimeter’s black probe. Next, link the red probe to the only wire from the sensor.

Ensure you adjust the multimeter to the ohm/resistance setting in a range corresponding to the ratings of the specific sensor in your vehicle (usually 20kΩ).

Step 4: Measure the Hot and Cold Readings

Dip the sensor’s probe into cold water and check the multimeter readings. 

Since it is an NTC thermistor, the sensor resistance reading will increase. Switching to warm or hot water should decrease the readings.

How To Test a Single-Wire Coolant Temperature Sensor: An engine’s radiator hose with the coolant temperature sensor mounted on it

An engine’s radiator hose with the coolant temperature sensor mounted on it

You can also switch to voltage readings in the 20V range. The voltage should increase when dipped in cold water to about 3-4V. 

In hot water, the voltage reading should decrease to somewhere between 0.5 and 1.2V.

A 5V reading usually implies an open circuit. Check the wire-to-ground link or signal wire connection. 

A 0V reading indicates a short circuit. Check the wires, as well.

If the readings don’t change, the sensor is faulty.

Wrap Up

Single-wire coolant temperature sensors are easy to test because they feature fewer wiring than the 2-wire and 3-wire types. 

So, you can do this test as a DIY project and even replace the unit if it is faulty.

While at it, you can test the temperature gauge to ensure all components are healthy. 

Comment below to share your testing experience, and let us know if you experience challenges.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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