Your car’s coolant temperature sensor (CTS) plays a vital role in keeping your engine running at the correct temperature. This small but essential component communicates with the engine control unit (ECU), helping it adjust fuel delivery, ignition timing, and cooling fan operation based on temperature readings. But when the sensor malfunctions, it throws your engine off balance.
A failing coolant temp sensor can cause overheating, poor fuel economy, hard starts, rough idling, and erratic temperature readings. Worse, if left unchecked, it will cause your ECU to make improper adjustments, leading to engine damage or expensive repairs.
Signs Of a Bad Coolant Temp Sensor
Diagnose a Bad Coolant Temperature Sensor (CTS)
How to Replace a Coolant Temperature Sensor (CTS)
In this guide, we’ll break down:
- The key symptoms of a bad coolant temp sensor
- Common causes of failure
- How to diagnose the issue
- A step-by-step replacement guide
Signs Of a Bad Coolant Temp Sensor
1. Poor Fuel Economy
A faulty CTS can send incorrect temperature readings to the engine control unit (ECU), making the system believe the engine is colder than it actually is. As a result, the ECU injects more fuel than necessary to compensate for the “cold” condition, causing the engine to run rich. Over time, this reduces fuel efficiency and increases fuel costs.
How to Tell:
- You notice you’re filling up more often than usual.
- Strong fuel smell from the exhaust, indicating excess fuel.
- The check engine light (CEL) may turn on, showing fuel-related trouble codes.
2. Hard Starting or No Start
A bad coolant temp sensor can cause starting issues because the ECU relies on accurate temperature readings to determine the correct fuel mixture for startup.
- If the sensor falsely reports a cold engine, the ECU may dump too much fuel into the cylinders, flooding the engine and making it hard to start.
- If it falsely reports a hot engine, the ECU may withhold necessary fuel, causing a lean condition and making it difficult to start.
How to Tell:
- Takes longer to start than normal, especially in the morning.
- Repeated cranking before the engine finally turns over.
- Excess fuel smell if the engine is flooded.
3. Rough Idle & Engine Stalling
An incorrect coolant temperature reading can disrupt the air-fuel mixture, leading to rough idling or even engine stalls. When the mixture is too rich or too lean, the engine struggles to maintain a stable idle.
How to Tell:
- Engine shakes or vibrates when idling.
- You experience random engine stalls when stopping at lights or in traffic.
- The RPM needle fluctuates unexpectedly.
4. Overheating & Check Engine Light (CEL) On
Since the CTS triggers the cooling fan, a faulty sensor might not activate the fan at the correct temperature, leading to engine overheating.
How to Tell:
- Temperature gauge creeps into the red zone.
- Warning lights on the dashboard for overheating.
- The CEL turns on, and an OBD-II scanner may show trouble codes like P0115 – P0119, indicating sensor failure.
5. Cooling Fans Running Constantly
A failing coolant temp sensor can also misinterpret engine temperatures, leading to the cooling fan running non-stop, even when the engine is cold. This can cause unnecessary wear on the fan motor and waste power.
How to Tell:
- Cooling fans keep running long after the engine is turned off.
- Engine never seems to warm up properly, leading to poor fuel economy.
6. Black Smoke from the Exhaust
When a CTS sends incorrect cold temperature readings to the ECU, the system compensates with extra fuel. The excess fuel doesn’t always burn completely, and some ends up exiting through the exhaust. This can lead to thick black smoke, and the excess fuel in the exhaust can damage the catalytic converter and these guys on the spendy side to replace.
How to Tell:
- Black smoke from the tailpipe, especially during acceleration.
- Unburned fuel smell around the vehicle.
- Possible carbon buildup on spark plugs from excess fuel.
7. Temperature Gauge Malfunctions
The CTS provides data to the engine temp gauge on the dashboard. If the sensor fails, the gauge may act erratically or not work at all.
How to Tell:
- The temperature gauge stays at “cold” even after driving for a while.
- The gauge suddenly spikes to hot, even if the engine isn’t overheating.
- Fluctuating readings—one minute it’s normal, the next it jumps up or drops.
8. Engine Misfires & Poor Performance
When the ECU receives bad data from the CTS, it struggles to adjust the air-fuel ratio and ignition timing properly. This can cause misfires, hesitation, and sluggish performance.
How to Tell:
- Sluggish acceleration, especially when the engine is cold.
- Hesitation or jerking when trying to accelerate.
- Misfires, leading to engine shaking or loss of power.
Diagnose a Bad Coolant Temperature Sensor (CTS)
Diagnosing the CTS correctly ensures you don’t replace parts unnecessarily. Below are three effective methods to test and confirm if your coolant temp sensor is faulty.
1. Use an OBD-II Scanner to Check for Trouble Codes
Since the CTS is connected to the ECU, a failing sensor often triggers a check engine light (CEL). The quickest way to check for a faulty CTS is by using an OBD-II scanner to read stored trouble codes.
Steps to Diagnose with an OBD-II Scanner:
- Turn off the engine and locate the OBD-II port (usually under the dashboard).
- Plug in the scanner and turn the key to the ON position (without starting the engine).
- Scan for trouble codes and look for any of the following:
- P0115 – Coolant Temp Sensor Circuit Malfunction
- P0116 – Coolant Temp Sensor Circuit Range/Performance Issue
- P0117 – Coolant Temp Sensor Circuit Low Input
- P0118 – Coolant Temp Sensor Circuit High Input
- P0119 – Coolant Temp Sensor Circuit Intermittent
- P0125 – Insufficient Coolant Temperature for Closed-Loop Fuel Control
- P0128 – Coolant Temperature Below Thermostat Regulating Temperature
- If any of these codes appear, your CTS may be faulty or have a wiring issue.
Mechanics Tip:
- If you get P0128, it could also indicate a stuck-open thermostat; make sure to check both the CTS and thermostat.
2. Perform a Multimeter Test on the CTS
The coolant temperature sensor (CTS) is a thermistor, meaning its electrical resistance changes with temperature. However, not all CTS sensors behave the same way. There are two main types:
- Negative Temperature Coefficient (NTC) Sensors – These are the most common type. Their resistance decreases as temperature increases (i.e., cold = high resistance, hot = low resistance).
- Positive Temperature Coefficient (PTC) Sensors – These work the opposite way; their resistance increases as temperature increases (i.e., cold = low resistance, hot = high resistance).
Before testing your sensor, check your vehicle’s service manual to determine which type of CTS you have and what the expected resistance values should be. Different car manufacturers use different resistance ranges, so using the wrong specifications can lead to incorrect diagnoses.
Method 1: In-Vehicle Multimeter Test
This method allows you to test the sensor without removing it from the engine.
- Turn off the engine and allow it to cool completely.
- Locate the coolant temperature sensor (typically near the thermostat housing).
- Disconnect the CTS wiring harness to expose the sensor terminals.
- Set the multimeter to measure resistance (Ohms Ω).
- Take an initial resistance reading when the engine is cold.
- Start the engine and allow it to warm up.
- Observe the resistance reading:
- If you have an NTC sensor, resistance should gradually decrease as the engine warms up.
- If you have a PTC sensor, resistance should gradually increase as the engine warms up.
- Compare the readings to the manufacturer’s specifications. If the resistance does not change as expected or remains stuck at a high or low value, the sensor is likely faulty.
Mechanics Tip: If your sensor is hard to access, you can monitor its live data using an OBD-II scanner instead of a multimeter.
| NTC Coolant Temp (°F / °C) | Expected Resistance (Ω) |
|---|---|
| 32°F (0°C) | 9,000 – 12,000 Ω |
| 68°F (20°C) | 2,500 – 3,500 Ω |
| 104°F (40°C) | 1,000 – 1,500 Ω |
| 176°F (80°C) | 250 – 400 Ω |
Method 2: Off-Vehicle Test with Hot Water & Thermometer Probe
If you prefer to test the sensor outside the engine, you can use a water temperature probe and hot water for more precise results.
- Remove the coolant temp sensor from the engine.
- Prepare a container of hot water and use a thermometer probe to measure the water temperature.
- Submerge the sensor tip in the hot water and take resistance readings at different temperatures.
- Compare the resistance values with manufacturer specifications.
- If you have an NTC sensor, resistance should decrease as the water heats up.
- If you have a PTC sensor, resistance should increase as the water heats up.
- If resistance readings are out of range or do not change correctly, the sensor is faulty and should be replaced.
3. Check the Coolant Temperature Gauge for Erratic Readings
If your car has a temperature gauge on the dashboard, it can provide visual clues about a faulty coolant temp sensor.
Steps to Check the Coolant Temp Gauge:
- Start the car and observe the gauge.
- If the gauge stays on “cold” even after driving for 10–15 minutes, the sensor might be faulty.
- If the gauge spikes suddenly to “hot”, but the engine is not overheating, it could be a bad CTS.
- Look for fluctuating or inconsistent readings.
- A temperature gauge that moves up and down randomly indicates a failing CTS or a wiring issue.
- Compare gauge readings to actual engine performance.
- If the engine seems to run fine but the gauge is acting weird, the issue might be with the sensor, not the cooling system.
Mechanics Tip:
- Some cars have two coolant temperature sensors: one for the ECU and one for the gauge. Make sure you’re testing the correct one!
How to Replace a Coolant Temperature Sensor (CTS)
Replacing a coolant temperature sensor is a straightforward job that requires minimal tools. By preparing the new sensor in advance, you can quickly swap it out without needing to drain the entire cooling system.
Tools & Materials Needed:
- New coolant temp sensor (ensure it’s the correct type)
- New sealing washer (commonly copper)
- Wrench or deep socket (size varies by vehicle)
- Coolant (for topping up the system)
- OBD-II scanner (for clearing codes)
Step-by-Step Guide
1. Locate the Sensor
- The CTS is usually near the thermostat housing or on the cylinder head.
- Consult your vehicle’s service manual if unsure.
2. Prepare the New Sensor
- Inspect the new sensor to ensure it has the correct threads and comes with a new sealing washer.
- If a washer is needed, install it on the sensor before installation.
3. Remove the Old Sensor
- Unplug the wiring connector and inspect the pins for corrosion or damage.
- Use a wrench or deep socket to unscrew the sensor.
- Quickly swap in the new sensor to minimize coolant loss.
4. Install the New Sensor
- Thread the new sensor by hand to avoid cross-threading.
- Tighten snugly, but do not over-tighten—this can damage the sensor or cause leaks.
5. Reconnect the Wiring Harness
- Ensure the connector clicks securely into place.
6. Top Up Coolant
- Check coolant levels and top up if necessary.
- Start the engine and let it reach operating temperature.
- Check for leaks around the sensor.
7. Clear Trouble Codes & Test Drive
Use an OBD-II scanner to clear any stored CTS-related codes.
Take the car for a short test drive to verify that:
- The temperature gauge reads normally.
- The cooling fan cycles correctly.
- Check for leaks around the sensor.
- There are no check engine lights or driveability issues.
Coolant Temperature Sensor (CTS) FAQs
1. Can a bad coolant temp sensor cause transmission issues?
Yes, in some vehicles, the ECU uses coolant temperature data to adjust transmission shift points. If the sensor sends incorrect readings, it can cause harsh or delayed shifting. Some cars may even go into limp mode if the sensor fails completely.
2. Will a faulty coolant temp sensor affect my car’s A/C system?
Yes, vehicles disable the air conditioning system if the ECU detects an overheating condition, which can be caused by a faulty CTS. If your A/C suddenly stops working and the engine temperature readings seem off, the CTS could be to blame.
3. Can a bad coolant temperature sensor drain my battery?
Yes, if the CTS fails in a way that tricks the ECU into keeping the cooling fan running continuously, it can drain the battery when the car is parked. If your battery dies overnight, check if the cooling fan is still running after shutting off the engine.
4. Can I drive with a bad coolant temp sensor?
It depends on how the sensor has failed. A mild failure may cause minor fuel economy issues, but a severe failure can lead to engine overheating, hard starts, or stalling, which could leave you stranded. Driving too long with a bad CTS can also cause catalytic converter damage due to an excessively rich fuel mixture.
5. Does the coolant temp sensor control the thermostat?
No, the coolant temperature sensor does not control the thermostat—the thermostat is a mechanical component that opens and closes based on coolant temperature. However, the CTS does provide data to the ECU, which may adjust fuel delivery, ignition timing, and cooling fan operation based on temperature readings.
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- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.