Signs Your Engine Coolant Temperature (ECT) Sensor Is Bad

Mia Bevacqua
June 29, 2018

Engine efficiency, power and fuel economy all improve as the engine temperature rises. Ideally, the engine will run as hot as possible without boiling its coolant or damaging its components. 

But a faulty engine coolant temperature (ECT) sensor can cause havoc on an engine or its related systems. These are the most common problems:

1. Your check engine light is on: If the engine’s computer detects a problem with the coolant temperature sensor, or its circuit, it will turn on the check engine light.

2. Your engine stalls: When the engine is first started, it will require more fuel in order to idle smoothly until it’s warmed up. The signal from the coolant temperature sensor tells the engine’s computer when to apply extra gasoline during a cold start. A faulty sensor can confuse the computer, keeping it from providing enough fuel. As a result, the engine may hesitate or stall.

3. You’re getting poor gas mileage: Until the vehicle is warmed up, the car’s computer relies on information from the coolant temperature sensor for fuel control. Once the car is warm, it switches to relying on the oxygen sensors for a more precise reading. A failed ECT sensor can prevent this change from happening, resulting in a rich-running engine and poor fuel economy.

4. Your engine runs poorly: The sensor affects almost every aspect of engine management. A bad one can easily cause a misfire or poor engine performance.

5. Increased emissions: If the sensor sends a faulty “cold” reading, the engine will run rich. As a result, carbon monoxide emissions will increase, and possibly hydrocarbons, as well. On the other hand, if it sends a faulty “hot” reading, the engine may misfire and run rough, increasing hydrocarbon emissions.

6. Your engine is overheating: If the engine’s computer doesn’t receive the correct temperature reading, it may not slow the ignition timing and turn on the cooling fans when necessary, causing the engine to overheat. If this happens, pull off the road and shut the car off as quickly as possible, as overheating can quickly kill your engine.


What is the ECT sensor?

The ECT sensor, also commonly just called the coolant temperature sensor, measures how hot the engine in your car is. Fuel delivery, ignition timing and engine performance are among the key elements affected by the information the coolant temperature sensor gathers. 

Typically, the coolant temperature sensor is mounted in the engine block or cylinder head, near the thermostat or where the coolant flow is hottest. More than one coolant temperature sensor may be used on some systems to determine the temperature in different locations. 

The tip of the sensor is submerged in coolant. The amount of voltage returned by the sensor to the engine’s computer increases along with the temperature of the coolant. This way, the computer knows how hot the engine is running and if it’s getting close to overheating.

The ECT sensor may also control the temperature gauge on your dash, though in some vehicles, that is done instead by the engine coolant sending unit, otherwise known as a temperature switch. 

» LEARN MORE: Get an estimate for your coolant temperature sensor replacement

How to fix the problem

Before spending time and money to replace the coolant temperature sensor, a thorough diagnosis should be performed. A professional can do this using equipment such as a scan tool, digital multimeter or oscilloscope.

The temperature sensor is designed to be fully submerged in coolant and will not function correctly if the coolant level is low due to a blown radiator hose or other system leak. So, check to make sure there’s no coolant leak before getting a new sensor.

A faulty sensor should be replaced. Because the ECT sensor is submerged in coolant, the cooling system should be drained at this time and then refilled. If the coolant has been contaminated, you should consider having the system flushed.

On some cars, the sensor can be easy to access. But since the sensor is so vital to engine health and performance, it’s a good idea to have a trained technician perform these repairs.


Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

2 User Comments

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By , December 31, 2010
You really need to check your raditor cap, they told me it was they sencor changed still same problem changed cap fixed problem purchased it at advance auto $8.00
By , March 04, 2019
My temperature gauge is stuck on being extremely hot even when truck is off

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