7 Symptoms of a Bad Coolant Temp Sensor (and Replacement Cost)

Today’s vehicles rely on complex network of sensors which monitor engine efficiency. But when just one sensor fails, an array of concerning symptoms can emerge.

The engine coolant temperature sensor is especially vulnerable. Its failure can lead to severe issues. Let’s look at the common symptoms of a faulty coolant temp sensor, why it fails, and its typical replacement cost.

faulty coolant temp sensor

Bad Coolant Temperature Sensor Symptoms

With time, coolant temperature sensors in many vehicles have a tendency to fail. In most cases, this comes as a result of continuity loss within the sensor itself.

When this occurs, a false temperature value is sent to the vehicle’s ECM, which leads to improper system response. As a result several symptoms often present themselves in short order.

The following are 7 of the most common symptoms associated with a failing engine coolant temp sensor.

#1 – Check Engine Light

check engine light

One of the most common symptoms associated with engine coolant temperature sensor failure is the sudden appearance of a check engine light. By its nature, a vehicle’s on-board diagnostics system stores trouble codes associated with many component failures and electrical circuit related problems.

If a vehicle’s ECM detects the delivery of irrational data from one of its sensors, a check engine light is illuminated. This light will remain on until the offending code is cleared, or the corresponding malfunction is remedied.

Related: DTC P0115, DTC P0116, DTC P0117, DTC P0118, DTC P0480

#2 – Poor Fuel Economy

bad fuel economy

Another symptom commonly associated with the failure of a coolant temperature sensor is poor fuel economy.

This stems from the fact that coolant temperature sensor feedback plays a vital role in the calculation of fuel delivery and timing. More often than not, a faulty engine coolant temperature sensor will incorrectly reflect a cool temperature reading.

When a permanently cool signal is sent by a coolant temperature sensor, a vehicle’s ECM attempts to compensate for this condition by dosing more fuel than is needed. In the end, this is reflected by more frequent trips to the pump.

#3 – Dark Colored Exhaust

black smoke from exhaust

As previously mentioned, a faulty coolant temperature sensor that has defaulted to a permanently cold reading will typically cause an engine to over-fuel (aka: run rich). As a result, excess unburnt fuel is passed downstream, where it often ends up entering the exhaust system.

When this excess fuel burns within a vehicle’s exhaust system, dark grey or black colored smoke is produced as a byproduct. In severe cases, this smoke can become very dense, and quite noticeable to anyone nearby.

#4 – Engine Overheating

steam from radiator

One of the most severe symptoms of coolant temperature sensor failure is engine overheating. While some vehicles utilize an auxiliary temperature sensor to regulate fan operation, the vast majority of makes and models rely upon the engine coolant temperature sensor to sustain this function. 

If an engine coolant temperature sensor of this type fails, it is common for the vehicle’s radiator fan to be inoperable. As a result, engine temperatures begin to spike, risking damage to other components.

Related: Losing Coolant But No Visible Leaks? (Here’s What it Means)

#5 – Fluctuating Temperature Gauge

If the coolant temp sensor isn’t working correctly, it can provide inaccurate readings, affecting your temperature gauge and making it fluctuate.  This might lead you to think your engine is overheating when it’s not, or vice versa.

Stay aware, as these wrong readings can cause unnecessary panic or even lead to potential engine damage if an actual overheating issue goes unnoticed.

See Also: Temperature Gauge Stuck? (Here’s Why)

#6 – Rough Idling

Due to incorrect temperature readings, your vehicle’s computer may adjust the air-fuel mixture improperly, causing an uneven and rough idle. It’s essential to keep an eye on this, as it could make your driving experience less comfortable and may lead to more significant issues.

#7 – Hard Start at Certain Temperatures

car won't start

Though not as common as other symptoms, a hard start condition is occasionally presented by a faulty coolant temperature sensor. In most cases, this condition will present itself only when a vehicle’s engine is at one particular temperature, whether hot or cold.

On most occasions, this is the result of a sensor which is providing a continually hot reading, even when an engine is actually cool. As a result, this typically renders a lean condition, which exponentially increases crank times.

Engine Coolant Temperature Sensor Replacement Cost

Best places to order parts?  See: 19 Best Online Auto Parts Stores

coolant temp sensor replacement cost

The cost associated with coolant temperature sensor replacement can vary significantly from one model of vehicle to the next.

While the coolant temperature sensors found on many vehicles are relatively simple to replace, others can be quite difficult to access. The actual cost associated with the purchase of a coolant temperature sensor can also vary by vehicle.

On average, the cost of a coolant temperature sensor ranges from $20-$80, while the cost of labor ranges from $50-$250. In total, one could expect to pay $70-$330 to have their engine coolant temperature sensor replaced.

How an Engine Coolant Temperature Sensor Works

Internal combustion engines rely upon the use of a type of coolant to regulate operational engine temperatures. Coolant cycles through an engine’s cylinder heads and block, where it picks up heat that is produced as a byproduct of the combustion process.

Coolant is then directed to the radiator, where incoming airflow dissipates heat stored within the coolant. As its name would suggest, a vehicle’s coolant temperature sensor monitors the temperature of the coolant flowing throughout an engine.

This information is then relayed to the vehicle’s ECM (Engine Control Module), where it is used as an input when determining how best to regulate various engine functions. These functions include fuel metering, EGR operation, and fan engagement. This data is also displayed to the driver via a temperature gauge in the dashboard.

For a more technical explanation of how the sensor works, check out this video:

Where is the Engine Coolant Temperature Sensor Located?

In most vehicles, the coolant temperature sensor can be found near the engine thermostat. On many late-model vehicles, this sensor is often threaded into the thermostat housing itself.

Now, the thermostat housing is usually located on the cylinder head, close to the intake manifold. This part of the engine is designed to help regulate the circulation of coolant within your vehicle’s cooling system. So, it makes sense that the coolant temperature sensor would be placed close by. It’s the best spot for capturing accurate temperature readings from your coolant.

Keep in mind that some vehicles might have more than one coolant temperature sensor. In these cases, you’ll typically find one sensor near the thermostat housing, like already mentioned, and another one elsewhere, such as near the radiator or in the lower engine block.

This is to ensure that the engine control module (ECM) receives precise, real-time data on the whole system’s temperature fluctuations.

Causes of Sensor Failure

#1 – Low Coolant Level

One of the common reasons for a faulty coolant temperature sensor is a low coolant level. Suppose your coolant level is too low; it can cause air pockets to form in the cooling system, which leads to incorrect readings from the sensor.

In turn, this can affect the engine management system, resulting in issues like wrong fuel injection, poor fuel economy, and an overheating engine.

#2 – Air Pockets

As mentioned above, air pockets can lead to fluctuations in coolant temperature, which could cause erratic readings from the sensor. A cooling system flush might help eliminate the air pockets and improve sensor performance.

If you notice your cooling fans cycling erratically, one potential reason could be air in the system causing the sensor to send faulty signals.

#3 – Corrosion and Bad Connections

Over time, corrosion can build up on the engine coolant temperature sensor connections, causing a poor connection between the sensor and the engine management system. This weak connection can lead to incorrect readings and may trigger a trouble code.

#4 – Damaged Wiring and Connectors

Damage to wiring and connectors can also lead to a malfunctioning coolant temperature sensor. The coolant temperature sensor is connected to the engine management system using wiring and plastic hoses. If the wiring or connectors are damaged, this can cause incorrect readings from the sensor, and potentially even total failure of the sensor.

In these situations, it’s necessary to inspect all related wiring and connectors for damage and replace any damaged components.

Can You Drive With a Faulty Coolant Temp Sensor?

While a vehicle with a faulty coolant temperature sensor can often be driven from one location to the next, doing so is not advised. With time, many of the individual symptoms of a failing coolant temperature sensor can become problematic themselves. 

An inoperable fan caused by a faulty coolant temperature sensor will quickly result in engine overheating. As a result, additional problems, such as head gasket leaks, can arise.

Likewise, abnormally high fuel rates caused by skewed data from a faulty sensor, can lead to the superheating and failure of a vehicle’s catalytic converter.

How to Diagnose a Bad Coolant Temperature Sensor

So, you’ve been noticing some weird stuff going on with your car’s engine temperature? Maybe it’s time to check out that coolant temp sensor. Don’t worry, diagnosing a bad coolant temperature sensor is something you can do. Let’s walk through it step by step.

#1 – Check Coolant Level

First things first, make sure your coolant level is good. You don’t want to misdiagnose a simple low coolant issue as a faulty sensor. Just pop the hood, locate the coolant reservoir, and check the level. If it’s low, top it off and see if your engine temperature issues persist.

#2 – Inspect Cooling Fans

Now, if your coolant level is fine, it’s time to check out the electrical cooling fans. These bad boys help keep your engine cool, and if they’re not coming on at the proper time, your engine temperature can skyrocket.

Start your engine and watch the cooling fans. They should kick on when the engine reaches a certain temperature or when the AC is turned on. If they don’t come on as expected, it might be the coolant temperature switch causing the problem.

#3 – Scan for Trouble Codes

Grab your trusty scan tool and connect it to your car’s OBD-II port. This little gadget will let you know if there are any stored trouble codes related to the coolant temperature sensor. If you see one or more codes, take note of them and do some research on what they mean. These codes can help you pinpoint the issue.

At this point, if you suspect the coolant temperature sensor is the issue, you should consider having a mechanic take a closer look if you don’t feel confident in replacing the sensor yourself. The pros will have the proper tools and expertise to accurately diagnose and replace the faulty sensor if needed.


Can a Bad Coolant Temp Sensor Affect the Thermostat?

Yes, a malfunctioning coolant temperature sensor can affect the thermostat in your vehicle. The coolant sensor is responsible for sending temperature information to the engine control unit (ECU).

If it’s not functioning correctly, it can send incorrect temperature readings, causing the thermostat to open or close at the wrong time. This can lead to your engine overheating or running inefficiently, affecting the overall performance and fuel economy of your vehicle.

How Many Coolant Temperature Sensors Does a Car Have?

In most vehicles, there is usually only one coolant temperature sensor responsible for monitoring the engine’s temperature. However, some vehicles may have multiple sensors in different locations to monitor the temperature of specific parts in the cooling system, such as the radiator or the heater core.

Having multiple sensors helps the ECU to regulate the various functions related to the cooling system more effectively, such as controlling the cooling fans, thermostat operation, and even the heating and air conditioning systems.

Josh Boyd


  1. My temperature guage only oscillates slightly higher & then back normal almost immediately only when driving at speed; once only it climbed into danger area very briefly only to quickly fall back, coolant normal when observed. No problem when driving in town or in slow traffic, only when going at continual speed does the problem arise. John.

    1. It sounds like the cooling system is being overloaded and the radiator can’t keep up. Make sure there is no debris blocking the radiator. Also check between the condenser and the radiator.

  2. I have a Toyota Avensis 2011 model manual transmission which my RPM Tachometer stop functioning and start after some days later stop. While this happen it also stops the AC, the Radiator Fan too will stops and the engine vibrates and this give me setbacks driving the car. I try to scan with obd2 but it doesn’t connect when the Tachometer is working to show me engin rotational speed but if it come up the obd2 will connect. Kindly help me with the fault.

    1. I don’t know what the issue is, but it sounds like it might be an electrical problem. This could be a bad ground or damage to the wiring harness. I would check a wiring diagram and see if there’s any part of the wiring harness that is shared by all of those components.

  3. My 2015 Chevy Traverse has these symptoms. However, the fan kicks on randomly, and stays on. My question is… if the heat shield has been removed from the wires, can the wires get fried and result in a faulty coolant temp sensor?

    1. Generally a bad coolant temperature sensor throws a code. Do your fans kick on while you’ve been idling for a while? That is completely normal. There’s no airflow across the radiator so the fans have to do the job when you’re not moving forward.

  4. 2015 Buick Enclave, I have had a myriad of issues lately with the most irrigating being the a/c when engaged the button blinks 3 times and doesn’t turn on the a/c. Research indicates coolant temp sensor that I have replaced with the same result. Further research states an ignition switch can cause problems with accessories too. It is a quick repair and cost $35 or less pending the type. I did replace it along with swapping out 2.5 year old battery for a new one. This did remedy intermittent voltage spikes to the ignition coil and the engine is running smoother but the a/c is still failing at times. When it does work, it will run you out of the car but the intermittent failure gets old quick here in Houston Texas. Any thoughts to a remedy for this issue? I have heard to check cabin filter and possibly evaporator issues, let me know what you think.

  5. In my 2003 Camry, my car would randomly shut off while driving(only while slowing down to a light) and a the code p0118 would appear. I replaced the thermostat sensor first, then alternator and battery as a mechanic suspected that could do it. (Battery was bad and was under warranty). Was still happening, replaced the thermostat and after a week the engine light came on and and shut off twice, but hasnt happened for a month or two.
    The temperature gauge will say -40C(which indicates either a faulty wire or computer) when driving when suddenly it will start working again and say the temperature is 90C (which it should be) after a few minutes of driving the temp will say -40C again. question is, it would not be a faulty wire as that would make it stay at -40C correct? so it would have to be something with the computer? correct?

    1. Sounds like a bad wire or ground. A bad coolant temperature sensor shouldn’t cause your car to shut off like that. Check the wiring that is shared between the fuel injectors and the coolant temperature sensor. Perhaps they share a common ground, or you may find an obvious short or problem with the wiring.

  6. Have a 2006 audi s4 with the 4.2 v8 engine . The car runs fine at cold start up and once it warms up it throws a code 0300 , 0301, 0304, 0305 , and 0308 and 0431 and 0421 cat effitioncy bank 1 and2 would the coolant sensor cause all these codes if it’s bad ?

    1. That is a lot of misfires. I highly doubt the coolant temperature sensor would cause this. A bad coolant temperature sensor will often throw a single code, and it’s probably not going to be a misfire.

      That sounds more like an issue with the fuel or ignition systems to me. Check to make sure you’re running the correct fuel and make sure your ignition coils are good.

  7. I’ve replaced several sensors and still have these 5 systems, just ordered it hope it fixes my issues finally, thank you so much for your details of exactly what my car is doing.

  8. My camry 1994 model is raising idle speed when the coolant sensor is connected. When I disconnect the idle comes down but engine idling becomes erratic. What could be the problem?

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