Sensors are your vehicle’s nervous system. In the same way your body is constantly monitoring your internal temperature, breathing, and more, your car is continuously monitoring data from a wide range of sensors. The engine coolant temperature sensor is one of the more important sensors in your car. Its job is fairly simple, but the information it provides is crucial for efficiency.
So, what happens when such an important part of your car’s nervous system fails? In this guide, we will show you exactly how to detect and diagnose ECT sensor issues.
What is an Engine Coolant Temperature Sensor?
The role of this sensor is to monitor the temperature of the coolant and send that information back to the engine management computer. It works in a rather simple manner.
The internal resistance of the sensor decreases as the temperature rises, reducing voltage. The ECU then reads the voltage, and it will adjust fuel injection timing and firing angle based on that.
What Happens When an Engine Coolant Temperature Sensor Goes Bad?
The engine coolant temperature sensor’s job is to ensure the engine operates at the ideal temperature. However, a sensor failure alone will not cause your engine to immediately grenade because it is not involved in the supply of coolant to your engine but can cause other issues if left unchecked.
That being said, the ECTS sensor does control the radiator fans in some vehicles. Cars usually have two temperature sensors where one does the job of the ECTS, while the other controls the cooling system. However, when there’s only one sensor in play, your fans may not kick in when they should. While rare, this issue can lead to engine overheating in situations where your radiator absolutely relies on your fans taking care of temperature management. Think rush hour traffic on a scorching summer day.
If you own a SAAB 9-5 for example, you’ll get a CEL on the dashboard and your temperature gauge will most likely act up. However, with these cars, there’s a roughly 50%-50% chance that it’s your ECTS or your thermostat. In other words, your problem could be more complex in nature.
What Are the Symptoms of a Faulty Temperature Sensor?
When sensors fail or malfunction, your ECU misses out on a bunch of often important data. This might not seem like a problem initially, but because the computer uses data to adjust actual car functions, the ECU could begin compensating in ways that are causing more harm than good.
Check Engine Light
A check engine light is the first indication that something isn’t right with your car. It doesn’t necessarily mean that your engine coolant temperature sensor has called it a day, but it’s a good signal that you should reach for your tools as soon as possible.
Getting a full readout of the ECU is a must whenever the orange light of doom pop up. Most cars will tell you that something is wrong, but not what exactly is wrong. In other words, that CEL can be something simple as a loose gas cap, or something as severe as ECU failure. Always check your CELs as soon as possible!
Incorrect Temperature Readings
This sensor works in combination with the thermostat. If it has failed, it can send faulty readings to your engine computer. In most cases it’s not responsible for the vital functions related to overheating, but your gauge might show hot or cold when the engine is operating normally.
There is an optimum temperature range in which the coolant made for your engine should run. Manufacturers test their coolants in this range and design them to deliver peak performance under these very specific circumstances.
A malfunctioning engine coolant temperature sensor can read the coolant as hotter than it is. Without the input from the ECT sensor, it will remain below the minimum threshold. Your car temperature gauge usually sits slap-bang in the middle, so look out for that when suspecting sensor failure.
Emissions Test Failure
Depending on your location, failing an emissions test can mean that you will no longer be able to drive your car, which no car owner wants to experience. Running richer often leads to increased carbon monoxide output which will present itself during the exhaust gas cycle test. Of course, many issues can cause increased emissions, so don’t jump to conclusions just yet.
This can also lead to excessive exhaust smoke as there will be plenty of unburnt fuel left due to a rich mixture.
Poor Fuel Economy
As the ECU uses the information from the sensor to adjust fuel trim, a failure can lead to the computer supplying more fuel than necessary. A richer air-fuel mixture doesn’t only mean inefficient combustion, it also means tangibly worse fuel economy. Ignored long enough, a bad ECT sensor can turn your otherwise efficient car into a true gas guzzler.
Increased Idle Speed
The fuel injection system receives data from multiple sensors, including the ECT sensor. If the engine management computer detects a richer air-fuel mixture caused by sensor failure, it will increase the engine idle so that it doesn’t stall.
Most engines idle at around 1000 rpm and 1500rpm on cold starts. If it is idling higher than that, it usually means there is an ECT sensor problem.
Stalling and Extended Cranking
Being unable to start your engine is the worst feeling a car owner can experience. A symptom of this is a long crank that leaves you wondering if the car will start again when you reach your destination? Truth be told no start or long crank conditions can be caused by a variety of other issues, but a bad ECTS shouldn’t be excluded.
Troubleshooting a Bad Engine Coolant Temperature Sensor
Sensor issues are often easier to diagnose as they communicate directly with the engine management computer. The best way to go about this is to run a full diagnostic scan using an OBD reader. However, a visual inspection of the sensor itself can tell you a lot about it as well.
Inspection Test
The quickest way to tell whether your engine coolant temperature sensor needs replacing is to check it for damage. The engine coolant sensor is usually located near the thermostat. Your car must be off before removing the sensor or touching any wires.
Take note of any melting, bent sensor pins, corrosion, or exposed areas. Examine whether the wiring harness is connected tightly to the sensor. Wire damage can also cause sensor issues, so check for those same issues on the nearby wires.
Fault Codes and the ODB II Reader
These days, your car can read faults from any part of the car, and you can interpret those using an ODB II reader. The trouble code for an ECT sensor is P0118.
Although this code can indicate a failure somewhere else in your cooling system, the sensor fault is usually the first to present itself. Inspecting your thermostat or radiator for damage sometimes involves removing parts which can get time-consuming.
Live data is one of the most practical tools in your arsenal when diagnosing an ECT sensor issue because the sensor output changes as your engine warm-up. Your ODB II reader can track this and note irregularities.
Connect it to your car using the ODB port. Most cars position it under or near the passenger glove box. Start your car. If the graph is spiking or dipping, or the readings are consistently low or high during testing can indicate sensor failure.
Resistance Testing
The engine coolant temperature sensor uses resistance to gauge coolant temp values. Because of that, a multimeter test will also tell you whether this part is working. Modern multimeters can measure resistance, amperage, and voltage, so a separate device is unnecessary if you already have a multimeter.
This method requires you to disconnect the sensor from the car. Double-check it is off before doing this. This sensor measures temperature, so the easiest way to test it is to simulate those conditions.
For this test, we intend to use a bucket of water to test the sensor’s ohm reading at cold-start temperatures. At room temperature, it should measure about 2 to 6 kΩ, or kiloohms.
Heat your water to about 190 °F (engine operating temperature) and measure the sensor again using the multimeter. It should read around 300 ohms.
Some cars use a PTC (positive temperature coefficient) resistance-based sensor. The readings above are for most vehicles that use an NTC (negative temperature coefficient).
Although they are a minority, cars that use a PTC resistor are also a part of our community. Consult your manual to see what type of sensor you have. If it is a PTC sensor, your readings should be around 250 ohms on a cold start and 380 ohms at operating temperature.
Voltage testing
Your sensor uses three wires, signal (red), live (green), and ground (brown). Always reference your car owner’s manual before testing, as the colors can differ between manufacturers.
Apply your positive (red) multimeter probe to the ECT sensor’s signal terminal and ground the negative (black) probe. The probe needs to make contact with the terminal to get a reading. It should read 2V when your engine is warm and 0.5V on a cold start. As the tolerance for this sensor is low, if it is over 0.2V above or below, the sensor is faulty.
A consistent voltage of 0 can mean that your battery is not supplying adequate power because of a sensor or battery failure. 5V consistently indicates that there is a break in the circuit somewhere.
Oscilloscope
You can use the same bucket method to test the sensor using an Oscilloscope. This tool will give you a clearer picture of whether the sensor is working.
Connect your oscilloscope to the sensor and set it to continuous measurement mode. Place the sensor in the water. Now you can measure the voltage using cold water and heating it or using hot water and letting it cool. The choice is yours.
The curve should be a smooth exponential one, between 1V and 4V, whichever way you decide to test it.
Can I Drive With a Bad Engine Coolant Temperature Sensor?
We would not advise you to drive with a faulty engine coolant temperature sensor. Running your car rich can lead to excess carbon buildup, which leads to its own set of serious issues.
The sensor is inexpensive and quick to replace compared to any engine damage that its failure can cause. It’s best to get it done as soon as possible.
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