How to tell if your Coolant Temp Sensor is Bad

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Alright, let’s dive into the world of car mechanics with a modern twist. Picture this: you’re on a road trip, the sun is shining, your favorite song is playing, and suddenly, your car starts acting up. It’s overheating, or the fuel economy isn’t what it used to be. Could it be the coolant temperature sensor? Let’s find out!

Your vehicle has many sensors that can trigger the check engine light, but the coolant temp sensor is the most common culprit. Its job is to inform the engine computer how hot the engine is. The sensor is part of the cooling system, which circulates coolant throughout the engine to absorb heat. The engine computer uses data from this sensor and other sensors to determine how much fuel is required for optimal performance and efficiency. If the coolant temp sensor is faulty, your car may not run properly or leave you stranded. It’s essential to keep an eye on this sensor to avoid problems that can affect other engine parts, such as overheating, loss of power, decreased gas mileage, and timing issues.

What Does the Coolant Temperature Sensor Do?

Simply put, it’s a sensor that tells your car if the engine is running hot or not. If, for some reason, this sensor does not work or is faulty, then your cooling system will start malfunctioning and may cause severe damage. This sensor sends an electrical signal that varies depending on whether the engine is hot. Checking up on this sensor occasionally will allow you to avoid any potential problems before they get out of hand.

The CTS is responsible for telling the ECU (Engine Control Unit) what temperature the coolant is. This is important because when the temperature is too hot, it will send a signal to the ECU that shuts down your car’s fuel injection system. This is to prevent any further damage from overheating. So, if this sensor stops working, your vehicle could be at risk of overheating, which can cause severe problems for other parts of the engine.

Symptoms of a Bad Coolant Temp Sensor

The following are a few ways to check if your coolant temp sensor needs to be fixed.

Symptoms of a bad CTS are wide-ranging, but here are some things you may experience:

Check Engine Light

The dashboard check engine light warns that your CTS may be harmful. The check engine light will come on until you take your vehicle to a professional mechanic. The check engine light means that the coolant temp sensor has sent out an error code that either means something is wrong or not working correctly.

Poor Mileage

Your engine’s fuel efficiency depends on the heat level it produces. If you have a faulty coolant temp sensor, then that means that your car’s temperature needs to be measured correctly, which results in lower mileage due to less efficiency.

Electrical Cooling Fans are not coming on.

The fans in the engine responsible for cooling the engine may only turn on if the coolant temp sensor is suitable because it can’t effectively measure how much heat is in the machine.

Black Smoke from the Exhaust Pipe

If you see smoke coming from your exhaust pipe, it is a sign that your engine is producing too much heat and the coolant temp sensor has failed to detect it.

Difficult Starting Condition

When turning the ignition key, the engine may be difficult to start or even turn on. This is because the coolant temp sensor influences your car’s ignition system regarding the air/fuel mixture.

Engine Overheating

If the coolant temp sensor doesn’t work correctly, your car will run hotter than it should because the sensor cannot detect the heat level properly, which ends up overheating your engine. When this happens, your car’s radiator fan may activate, but your cooling system can still become overheated if it begins to malfunction.

Poor Idling

If your engine is overheating while idling, more than likely, the coolant temp sensor has stopped working or failed. An idling car should not be getting hot, but if it is, there is an underlying issue that needs to be fixed before you end.

Poor Engine Performance

If there are problems with the cooling system, this will lead to an increase in heat which will cause loss of power, hesitation, and stalling. This situation can be hazardous if not handled, as it may lead to engine failure.

Coolant Temperature Sensor Location

The sensor itself can be found in the high-temperature coolant hose, which is located between the thermostat housing unit and the engine, or it can also be located in one of two places. The first place would be near the cylinder head, close to where the intake manifold meets it, while another location for this sensor is under your car’s hood on top of your engine block.

The temperature sensor needs to be checked occasionally, no matter what type you have, because they are all prone to breaking down at some point. Checking your CTS regularly will save you time and money compared to waiting for something like overheating.

Coolant Temperature Sensor Replacement Cost

The replacement cost may vary depending on the amount of labor, but generally, it should cost around $100-$150 to fix. Labor costs will add onto this and can cause the total bill to be anywhere from $30 for just a primary replacement job upwards to $200 when extensive labor is performed.

Parts such as coolant sensors are only covered under warranty if they fail within the manufacturer’s mileage limit, so you might have to pay upfront to fix your car. When purchasing a new sensor, consider buying one made by an OEM supplier because they tend to last longer than generics.

How To Make the Coolant Temperature Sensor Last Longer?

Generally speaking, oil and water do not mix, but that is exactly what the sensor has to work with due to its location. To prevent this from causing problems then, you should clean any residue or make sure that you keep the reservoir topped off with coolant. To make the sensor work correctly, you should drive within the engine’s optimal temperature range for best results.

Oil Contamination

If there is a significant issue with oil in your car’s cooling system, then this will result in overheating and can damage other components besides just the CTS, which results in more repair costs overall. If there is an issue with this, check everything out immediately because it can lead to severe long-term damage to your car.

Coolant Contamination

Another issue you must avoid is coolant contamination because it can cause the sensor to fail and potentially damage other components around it. If there is a coolant problem, consider changing out all of the coolants to save yourself some hassle later. It’s better to be safe than sorry, so this precautionary measure may keep you from having a few hundred dollars worth of repair costs later on.

How to Tell if Your Coolant Temp Sensor is Bad

  1. Your car is overheating: This is the most common sign. If your car’s coolant temperature sensor is faulty, it might not send the correct temperature reading to the engine control unit (ECU). This can cause your vehicle to overheat because the ECU isn’t triggering the cooling fan correctly.
  2. Poor fuel economy: If your car seems to be guzzling more gas than usual, it could be due to a bad coolant temp sensor. The ECU uses the sensor’s data to adjust the air-fuel mixture. If the sensor sends incorrect data, the ECU might enrich the mixture unnecessarily, leading to increased fuel consumption.
  3. Check Engine Light is on. The ECU monitors the coolant temperature sensor. If it detects a problem, it will turn on the Check Engine Light. However, remember that this light can indicate many issues, so it’s best to get a diagnostic test to confirm.
  4. Black smoke from the exhaust: This is a sign that your car is burning too much fuel, which could be due to a faulty coolant temp sensor. If you see black smoke, get your vehicle checked immediately.
  5. Cold start problems: If your car has trouble starting on cold mornings, it could be because the coolant temp sensor sends incorrect readings to the ECU, causing it to adjust the air-fuel mixture improperly.

Remember, these signs could also indicate other issues, so a professional must check your car if you notice any of them. But knowing these signs can help you communicate more effectively with your mechanic and understand what’s happening under the hood of your car.

Overuse Of Cooling System Additives and Treatment Products

When using antifreeze, radiator stop leak, and corrosion inhibitors, ensure they are specifically made for your car’s cooling system. Using incorrect products can lead to issues with the CTS and potential harm to other parts, resulting in higher expenses in the future.

Not Maintaining Your Cooling System

To ensure the longevity of your CTS, it’s essential to regularly maintain your coolant system by removing any rust or debris that may have accumulated. Without sufficient additives, the water in the system can become corrosive and damage essential engine components such as gaskets, valves, and water pumps. Neglecting maintenance can result in costly repairs that could have been avoided with regular upkeep.


The Coolant Temperature Sensor monitors the engine temperature and sends data to your car’s computer. You can identify if it’s malfunctioning by observing symptoms such as a check engine light or an abnormal reading on the temperature gauge. It’s crucial to replace it as soon as possible to prevent further damage to your car’s cooling system. Regular maintenance is essential to avoid issues like oil or coolant contamination, overuse of additives, and expensive future repairs.


What is a coolant temp sensor?

The coolant temperature sensor is a device that measures the temperature of the coolant in your car’s engine and sends this data to the engine control unit (ECU).

How does a coolant temp sensor work?

The sensor works by changing its internal resistance in response to changes in temperature. The ECU interprets this resistance change as a temperature change.

How much does it cost to replace a coolant temp sensor?

The cost can vary depending on the make and model of your car, but generally, it’s a relatively inexpensive part to replace.

Can I drive my car with a bad coolant temp sensor?

While driving with a faulty sensor is possible, it’s not recommended. A bad sensor can lead to overheating, poor fuel economy, and potential damage to your engine.

How often should I replace my coolant temp sensor?

There needs to be a set schedule for replacing this sensor. It’s best to replace it when you notice any signs mentioned above or when a diagnostic test indicates it’s faulty.

Remember, knowledge is power. Understanding your car’s needs will save you time and money and inspire you to confidently take on the open road. So, keep learning, keep exploring, and keep driving!

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