The coolant temp sensor is one of the car engine management systems that monitor the temp of the radiator coolant. The coolant temp sensor functions perfectly by monitoring the coolant temp with the aid of an electrical circuit.
The sensor sends a signal to the car computer to make changes for gas calculation and engine timing for peak performance.
As the vehicle engine requires less gas when warmed and more gas when cold, the car computer will adjust the engine performance setting if it receives a signal that coolant temp is above the normal operating temperature to save the engine from catastrophic damages due to the engine overheating.
Because the coolant temperature sensor plays a critical role in the engine timing and calculation for optimal performance, a lousy coolant temp sensor can easily affect your engine’s performance.
Hence, you should always watch out for symptoms of a bad coolant temp sensor. In this article, we’ll discuss at length how to tell if your coolant temp sensor is bad, the signs and symptoms of a defective coolant temp sensor, and how to fix the problem.
What Is the Engine Coolant Temperature Sensor?
The ECT sensor measures the coolant temperature in the cooling system, which lets the PCM control the spark timing and fuel mixture.
When the engine is cold, the PCM adjusts the fuel mixture to be richer than normal to prevent the engine from stalling and stumbling.
When it’s warm, the engine gets a leaner-than-normal fuel mixture to maximize fuel economy while releasing the lowest possible exhaust emissions.
The ECT sensor is a high-authority sensor whenever the engine is cold because ignition timing is tailored according to the engine coolant temperature.
It also provides input for idle air control (IAC) position, oxygen sensor closed-loop status, on/off times for the canister purge, and idle speed.
Some vehicles (like those from Chrysler and General Motors) have a step-up resistor that broadens the ECT sensor’s range. This extra circuit gives the PCM a more accurate engine coolant temperature reading compared to having one circuit.
What Does a Coolant Temperature Sensor Actually Do?
If you’re new to engine electronics, the coolant temperature sensor might look like just another plug sticking out of the engine. But its job is far more critical.
The ECT sensor constantly measures temperature inside the engine’s coolant passages and reports that information back to the ECU. Based on this temperature, the ECU makes dozens of decisions every second:
- How much fuel to inject
- When to adjust timing
- When to turn radiator fans on/off
- How rich the mixture should be at startup
- Whether the engine is at safe operating temperature
- When to activate closed-loop operation
In simple terms, the coolant sensor is like the thermostat of your engine’s brain. When it lies, the ECU makes wrong decisions—often dramatically wrong.
Types of Coolant Temperature Sensors
There are two primary types of coolant temperature sensors commonly used in vehicles:
- Resistance Temperature Detectors (RTDs): These sensors change their electrical resistance based on the temperature of the coolant. The ECU measures this resistance to determine the coolant temperature.
- Thermistors: These sensors also change their resistance with temperature, but their resistance decreases as the temperature increases. Thermistors are generally more sensitive than RTDs.
Where Is the Coolant Temperature Sensor Located?
The coolant temperature sensor is usually small and easy to miss. It is attached to the engine where it can measure coolant temperature accurately.
Common locations include:
- Near the thermostat housing on the engine block
- On the cylinder head in some car models
- Close to the radiator inlet in certain vehicles
- Attached to the coolant passage for precise readings
Knowing its location helps you check or replace the sensor if it starts causing problems.
Symptoms of a bad coolant temp sensor
The most common symptoms of a bad coolant temperature sensor include poor fuel economy, cooling fans that fail to turn on, rough engine performance, engine overheating, and a check engine light appearing on the dashboard.
Below is a more detailed list of the most common bad coolant temperature sensor symptoms to look for.
#1. Check Engine Light.
One of the first signs of all coolant temperature sensor symptoms you will probably notice is that the check engine light will show up on your dashboard.
If the computer detects any problem with the sensor’s circuit, it will illuminate the check engine light, indicating that the car needs an inspection.
If you see a check engine light on your dashboard, it is time to check the trouble codes with an OBD2 scanner.
#2. Poor Mileage.
A faulty ECT sensor can send a false signal to the onboard computer, resulting in an incorrect air-fuel mixture. For example, a faulty sensor can send a signal indicating the engine is cold when it is not, and more fuel will heat the engine quickly.
This will cause the fuel economy to drop and decrease the engine’s performance.
#3. Electrical Cooling Fans not coming on.
Some cars use the engine coolant temperature sensor to control the electric cooling fans. In most cars, you have two separate temperature sensors for the fans, a dashboard gauge, and engine management.
However, if your car has a single sensor, a bad engine coolant temperature sensor may result in your fans not starting.
#4. Black Smoke from the Exhaust Pipe.
Due to an incorrect engine temperature signal, the ECU may enrich the fuel mixture to a point where the combustion process becomes difficult.
The excessive fuel will burn in the exhaust pipe and will produce thick black smoke.
#5. Hard Start Condition.
The amount of fuel getting injected into the engine is critical at the moment you start your car. If the air-fuel mixture is faulty, you might find your car difficult or impossible to start.
#6. Engine Overheats.
The cooling fan, which is behind the radiator grille, removes heat from the engine’s coolant. This fan is electrically controlled and relies on the signal from the onboard computer.
If the fan receives a false signal, the fan might not turn on, causing the engine to overheat. Some vehicles have a separate coolant temperature sensor for the fan, but many cars use the same sensor.
#7. Poor Idling.
Due to a faulty ECT sensor, the fuel mixture will adjust. This will cause the engine to vibrate or shake when the car is at low speed and lead to other power losses and strange behaviors.
The engine is very sensitive to wrong air-fuel mixtures at idle, and this is a condition when you might notice that something is wrong with your engine coolant temperature sensor.
#8. Poor Engine performance.
The engine coolant temperature sensor can have a hard impact on the air-fuel mixture. A bad air-fuel mixture can also cause the engine performance to drop drastically.
If you feel that the engine performance is not what it has been before, it may be due to a faulty engine coolant temperature sensor.
Causes of a Bad Coolant Temperature Sensor
The causes of a bad coolant temperature sensor can vary, but some common factors that can lead to its malfunction include:
#1. Electrical Issues.
Faulty electrical connections, damaged wiring, or a corroded connector can disrupt the signal between the coolant temperature sensor and the engine control unit (ECU). These issues can cause inaccurate temperature readings or a complete failure of the sensor.
#2. Sensor Damage.
Physical damage to the coolant temperature sensor, such as cracks or breaks, can impair its functionality. This damage can occur due to accidents, improper handling during repairs, or excessive vibration.
#3. Contamination.
Contaminants, such as dirt, debris, or coolant leaks, can affect the performance of the coolant temperature sensor. If these substances enter the sensor housing or cover its surface, it can lead to inaccurate readings or sensor failure.
#4. Coolant System Problems.
Issues with the vehicle’s cooling system can indirectly impact the coolant temperature sensor.
Coolant leaks, low coolant levels, or a malfunctioning thermostat can cause inconsistent temperature readings or affect the sensor’s ability to measure the coolant temperature accurately.
#5. Overheating.
Continuous exposure to high temperatures, particularly engine overheating, can degrade the coolant temperature sensor over time. Extreme heat can damage the sensor’s internal components, leading to incorrect temperature readings or sensor failure.
#6. Manufacturing Defects.
In rare cases, a coolant temperature sensor may have manufacturing defects that cause it to malfunction prematurely. These defects can include poor-quality materials, improper assembly, or calibration issues.
How to Test Your Coolant Temp Sensor?
Now that you know the symptoms, let’s get hands-on. This section provides a practical how to tell if a coolant temp sensor is bad guide, offering actionable tips for DIYers.
Always prioritize safety: ensure your engine is cool before working on any cooling system components.
#1. Visual Inspection.
Start with the simplest check. Locate your coolant temperature sensor. It’s usually threaded into the engine block, cylinder head, or thermostat housing, often near the upper radiator hose. Look for:
- Damaged Wiring: Check for frayed, cut, or corroded wires leading to the sensor’s connector.
- Loose Connections: Ensure the electrical connector is securely seated on the sensor. A loose connection can mimic a faulty sensor.
- Coolant Leaks: Any coolant leaking around the sensor itself could indicate a faulty seal or a cracked sensor housing.
Address any visible issues first, as they might be the root cause of your problems.
#2. Scanning for Diagnostic Trouble Codes (DTCs).
If your Check Engine Light is on, this is your next step. You’ll need an OBD-II scanner (On-Board Diagnostics, second generation). These are affordable and readily available at auto parts stores or online.
- Plug the OBD-II scanner into your vehicle’s diagnostic port (usually under the dashboard near the steering column).
- Turn the ignition to the “ON” position (engine off).
- Follow the scanner’s instructions to read stored codes.
Common DTCs related to the coolant temperature sensor include:
- P0117: Engine Coolant Temperature Sensor 1 Circuit Low
- P0118: Engine Coolant Temperature Sensor 1 Circuit High
- P0119: Engine Coolant Temperature Sensor 1 Circuit Intermittent/Erratic
- P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control
These codes are strong indicators of a problem with the sensor or its circuit.
#3. Live Data Monitoring.
Many OBD-II scanners, especially smartphone-based apps with Bluetooth adapters, can display “live data.”
This allows you to see what temperature reading the ECU is actually receiving from the CTS in real-time. This is one of the best how to tell if a coolant temp sensor is bad tips for accurate diagnosis.
- Connect your scanner and start the engine.
- Navigate to the “Live Data” or “Data Stream” section.
- Find the parameter for “Engine Coolant Temperature” (ECT).
- Compare the displayed temperature to your dashboard gauge. If there’s a significant discrepancy, or if the live data reading is stuck at an absurdly low (-40°F/C) or high (250°F+) temperature, the sensor is likely faulty.
- Watch the temperature rise as the engine warms up. A healthy sensor will show a steady, gradual increase. A faulty one might show erratic jumps, drops, or no change at all.
#4. Manual Resistance Testing (Multimeter).
This method requires a digital multimeter and is a more precise way to test the sensor’s internal resistance. The CTS is a thermistor, meaning its electrical resistance changes with temperature.
Most automotive CTS units are Negative Temperature Coefficient (NTC) thermistors, meaning resistance decreases as temperature increases.
Tools Needed: Digital Multimeter, Repair Manual (for resistance specifications), Heat Gun or Pot of Water (for heating the sensor).
Steps
- Disconnect the Battery: Always disconnect the negative battery terminal before working on electrical components.
- Locate and Disconnect Sensor: Carefully unplug the electrical connector from the CTS.
- Remove the Sensor: You may need a deep socket or a specialized sensor wrench. Be prepared for a small amount of coolant to escape; have a drain pan ready.
- Set Multimeter: Set your multimeter to measure ohms (Ω).
- Measure Resistance (Cold): Touch the multimeter probes to the sensor’s terminals. Record the resistance reading at room temperature.
- Heat the Sensor: Carefully heat the tip of the sensor (not the connector!) with a heat gun, or immerse it in a pot of hot water (use a thermometer to measure water temp).
- Measure Resistance (Hot): As the sensor heats up, continuously monitor the resistance. It should steadily decrease.
- Compare to Specifications: Consult your vehicle’s repair manual for the specific resistance values at various temperatures. If your readings deviate significantly from these specs, the sensor is bad.
This detailed resistance testing is one of the how to tell if a coolant temp sensor is bad best practices for definitive diagnosis.
How to fix coolant temp sensor problems?
You might think the best solution to this problem is to change the sensor. Well, sometimes, you don’t have to replace the sensor.
Therefore, it is imperative that you know how to test coolant temp sensors using a diagnostic machine, an oscilloscope, or a digital multimeter.
For instance, the coolant temp sensor will send false readings for a low coolant level. If this is the case, you will fix the problem by topping your coolant to the appropriate gauge.
Working under the hood can be messy, especially when working on an area you haven’t cleaned for long.
So, you need a mechanic glove, eye protection, and long sleeve shirt. Organizing and keeping your mechanic tools close by will save you time in completing the work.
Replacing a temp sensor is an easy task and can be done by anyone familiar with the ‘underhold’ components. Follow the below steps to replace a defective temp sensor.
#1. Locate the temp sensor.
The sensor is typically located near the radiator or on the thermostat compartment. In some vehicles, the temp sensor is located at the back of the engine, so you may need a touch light to locate it.
The thermostat housing is connected to the upper radiator hose; follow the upper hose to locate the thermostat housing and find the CTS on that area.
#2. Remove the electrical connectors.
The temp sensor is connected to the car computer via an electrical connector. You need to carefully unplug the connector as it can be brittle and break. Always inspect electrical connectors for corrosion and clean them if you find any.
#3. Unfasten and remove the sensor.
Coolant temp sensors are typically installed like a spark plug, so you need a long socket and ratchet handle to unscrew it. If the sensor proves stubborn to break free, do not apply much pressure.
Rather a squirt of break-free solvent will help slack it easily. Remember to remove the radiator cover before removing the sensor. Once the CTS is loose, unscrew it by hand. The antifreeze will leak from this spot, so ensure you refill it after all.
#4. Compare both sensors and install the new one.
place both sensors together and compare them. This is usually necessary if you purchase an aftermarket part. Head over to the CTS housing and clean the debris from the old sensor with a rag.
Insert the new CTS into the housing and thread it by hand. Once you have incorporated the sensor and ensure it didn’t miss any thread, tighten it with a torque wrench to the manufacturer’s specified range.
#5. Reinstall the electrical connectors.
Once you have tightened the sensor, the next thing is to reinstate the electrical connector. Ensure there is no corrosion on the electrical connectors and plug it back to the CTS.
Plug back any fastener on the sensor and make sure it’s well fitted. Refill the coolant and put back the pressure cap on the radiator. Start the engine and wait to see if the temp gauge is reading correctly.
#6. Test the Vehicle.
Test run the car to see if your efforts have finally paid off. Once you’re back from the test drive, recheck the coolant level and see if it has dropped and refill it. If everything is nice and smooth, give yourself a pat on the back.
FAQs.
Q: How long does a coolant temperature sensor last?
Defective coolant temperature sensors are common. If you fail to carry out regular and scheduled maintenance specified by your manufacturer, it can corrode the CTS endpoint that touches the coolant.
Coolant temp sensors can last as long as 100,000 miles. However, if your engine cooling system lacks proper maintenance, the sensor can fail early.
Q: Does the temperature sensor control the fan?
The temp sensor works by monitoring and measuring the coolant temperature. The sensor then sends the temperature readings to the ECU, and as the ECU receives the reading, it may cause the radiator fan to either switch on or switch off.
Q: Does the coolant temperature sensor affect AC?
In a proper system operation, the coolant temp shouldn’t affect the air-conditioning system.
In any case, since the radiator fan helps the condenser fan in cooling the AC condenser, if the ECU shut off the radiator fan as a result of false readings from the CTS.
The AC condenser will lack enough air to cool it down, thereby affecting the optimum functionality of the entire AC system.
Final Thought
Keeping an eye on your coolant temperature sensor can save you from a lot of headaches down the road. By recognizing the signs of a malfunctioning sensor early on you can avoid costly repairs and ensure your vehicle runs smoothly.
Remember to test the sensor if you suspect it’s acting up and don’t hesitate to inspect the wiring for any damage. Whether you choose to replace the sensor yourself or seek professional help make sure you weigh your options carefully.
Taking these steps will help keep your engine healthy and your driving experience enjoyable. Stay proactive and take care of your vehicle’s needs for the best performance.