Knowing the temperature is very helpful. If you are going outside, you look and see that it’s 28 degrees and you should grab a warm hat. While your car doesn’t wear cozy hats, it’s crucial that it constantly monitors the temperature of the engine while it’s on.
That’s the job of the engine coolant temperature sensor (CTS or ECTS). Coolant is also known as antifreeze, the fluid that helps keep the engine at an optimal operating temperature.
There are several things that the vehicle can do to change the temperature when needed, so the temperature data that the CTS sends to the ECU (car’s main computer) is critical.
Some vehicles also have a cylinder head sensor (CHS) which sits at the top of the cylinder and is not affected by loss of coolant since it isn’t submerged by coolant like the CTS is. This makes the CHS more reliable than the CTS.
How Does a Coolant Temperature Sensor Work?
The CTS uses electrical resistance to measure the temperature, which means that the CTS is a thermistor. The resistance (opposition to electrical flow) of the sensor changes proportionately with temperature – as the temperature increases, the electrical flow also increases.
The ECU sends the electrical signal through the CTS, measuring the voltage drop. This converts information about the electrical flow into a temperature reading.
With that information the ECU adjusts the fuel injection, ignition timing, and electric radiator cooling fans to maintain an optimal temperature. If the engine is cold, the ECU directs the air/fuel mixture to be more rich, or a higher proportion of fuel for the amount of air entering the engine.
If the engine starts to get too warm, the ECU will kick on the radiator fans. This is normal behavior when you are sitting at a long stop light on a hot day, for example. Some cars shut off the engine if it’s overheating to protect from engine damage.
The temperature information is also sent to the dashboard gauge which is usually next to the fuel gauge.
Symptoms of a Bad Coolant Temperature Sensor
All parts wear out eventually, and this sensor is no exception. It’s critical to address problems with the cooling system, because if the vehicle ends up overheating it can cost you an engine (which is very expensive and time consuming to repair).
1) Overheating Engine
An overheating engine should give off several warnings like a high temperature reading on the dashboard gauge and sometimes white “steam” coming out from under the hood (this is boiling coolant, which means it’s leaving the system – that’s bad!).
Not having enough coolant is a problem. Leaking coolant can also cause the engine to overheat if there is not enough reserve to properly cool the engine.
2) Poor Engine Performance
If the sensor is faulty it may send incorrect temperature information to the ECU which can lead to strange engine behavior, like overall “weakness” or sluggishness.
If the vehicle lags going uphill, is slow to accelerate, idles roughly, or is difficult to start especially when it’s already warmed up, it’s worth checking the CTS.
3) Increased Fuel Consumption
You may see that your fuel economy worsens considerably if the sensor is bad because the computer may be directing too much fuel to be injected into the cylinders.
4) Black Smoke from Exhaust
For the same reason, the vehicle may run too rich which makes the excess fuel burn up in the exhaust and causes other drivers to glare at you.
5) Failed DEQ Emissions Test
If too much fuel or an abnormal amount of byproducts are being expelled due to inefficient combustion, it will show up on an emissions test as something that needs to be fixed.
The CTS may be the culprit, though there are several sensors and gaskets that should be checked.
6) Inaccurate Temperature Gauge
If the engine temperature reading on the dashboard gauge seems wrong (for example, if the gauge reads “cold” when the car is thoroughly warmed up), it may be getting wrong information from the coolant temperature sensor.
7) Cabin Air Conditioner Stops Working
Many vehicles will put the car into “fail-safe” mode if overheating is detected. This may turn off the engine, run the engine cooling fans continuously, and disable the interior AC to enable the car to dissipate heat from the engine more effectively.
8) Check Engine Light is On
The dashboard “check engine” light illuminates when the ECU detects a problem and stores a code. If you see this along with any of the other symptoms, it’s worth checking the CTS.
Check Engine Codes for a Bad Coolant Temperature Sensor
If you scan your check engine light and notice any of the following codes, there is a chance your coolant temperature sensor may be bad.
P0115 Engine Coolant Temperature Circuit Malfunction
This code means the engine control unit (ECU) has detected an unusual signal from the coolant temperature sensor. This could be caused by a bad ground or bad wiring.
P0116 Engine Coolant Temperature Circuit Range/Performance Problem
P0116 means the range of values given to the ECU by the coolant temperature sensor does not match the expected range. In other words, the resistance in the sensor may read the same when the car is cold and when the car is warm.
To check this, grab a multimeter and measure the resistance in the coolant temperature sensor, both when the engine is cold and once it has reached operating temperature. The coolant temperature sensor must remain inside the engine block for this procedure, but the connector to the sensor should be disconnected.
If the resistance changes as expected, you may have a bad connection at the ECU. If the resistance does not change, you likely have a bad coolant temperature sensor.
P0117 Engine Coolant Temperature Circuit Low Input
P0117 indicates the ECU has detected an unusually long duration for a low voltage from the coolant temperature sensor. For instance, the voltage may measure below 0.1 volts for 2.5 seconds or longer.
P0118 Engine Coolant Temperature Circuit High Input
P0118 indicates the ECU has detected an unusually long duration for a high voltage. For instance, the coolant temperature sensor may read above 4.9 volts for 2.5 seconds or longer.
P0119 Engine Coolant Temperature Circuit Intermittent
P0119 indicates that the coolant temperature sensor is giving an inconsistent signal that should not be possible. It takes time to heat water. If the coolant temperature sensor gives a voltage that oscillates between 1 and 4 volts several times a second, this is obviously a bad signal.
A bad signal can be caused by electrical issues such as a bad ground or worn insulation inside the wiring harness.
P0128 Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
P0128 means the coolant temperature is registering as colder than the thermostat has the ability to regulate. Thermostats are mechanical devices that work regardless of what the ECU is showing, but this code could indicate a problem with the coolant temperature sensor.
This code may also mean your thermostat is stuck open. A thermostat that is stuck open may not allow the engine to come up to operating temperature. A closed thermostat seals the coolant inside the block long enough for the coolant to retain a bit of heat. Engines do not operate efficiently if they are too cold.
Related: How to Replace a Thermostat
Coolant Temperature Sensor Replacement Cost
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The sensor is usually fairly easy to access, often on the radiator or in or near the thermostat. It’s a good idea to have a trusted mechanic take care of the diagnosis and replacement since the coolant should be refilled and air bubbles removed after the part is replaced.
Some coolant temperature sensors may be found in the engine block, or in part of the cooling system plumbing.
The sensor itself is usually between $40 and $70 depending on the vehicle, and labor is between $200 and $400, though it could cost more if other things need to be done to the vehicle.
Some vehicles do have a second coolant temperature sensor, located elsewhere in the engine bay or in the radiator.
If you choose to try to replace the CTS yourself, always practice safety first and remember that the coolant system is under a great deal of pressure when hot. Never remove the coolant cap or anything else in the system until the car is completely cool!
If you see any of the above symptoms, check the sensor itself and all connections and wiring between the CTS and the ECU to make sure there isn’t another issue along that line of communication.
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