How to Check & Replace an Engine Coolant Sensor

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Can Bad Coolant Temperature Sensor Cause Car To Stall?

Symptoms of a bad sensor vary, but a check engine light will be the common denominator. It’s such an important sensor. The engine light will alert the driver as soon as the onboard computer senses a problem.

As you know, the sensor is central to how the engine control modules make decisions about fueling, a flawed reading will cause poor running, and stalling is high on a list of symptoms.

Other common symptoms include:

  • Hard on gas
  • Strong smell of gas
  • Rough running
  • Hesitation on acceleration
  • High or low idle
  • Misfiring
  • Black smoke
  • Bad smell from the muffler
  • Overheating engine
  • Fan running constant
  • HVAC blows cool air

What Does the Coolant Temperature Sensor Do?

Basically, your engine coolant temperature sensor (CTS or ECTS) monitors the temperature of the coolant circulating through your car’s engine. In turn, this gives an idea of how hot your engine is. If your engine is overheating, it’s up to the coolant temperature sensor to pick up on that information and trigger the dashboard light, as well as any other responses your car is set to carry out.

Many modern cars will automatically shut off the engine once it reaches a certain temperature to protect it from excessive damage. If your coolant temperature sensor isn’t doing its job properly, the engine may not receive the signal to shut off  and you might find yourself facing serious engine damage.

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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.

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COOLANT SENSOR DIAGNOSTIC FAULT CODES

On 1996 and newer vehicles with OBD II onboard diagnostic systems, a faulty coolant sensor may prevent some of the system monitors from running. This will prevent the vehicle from passing an OBD II emissions test because the test can't be done unless all the required system monitors have run and passed.

The OBD II system should catch the fault, turn on the Check Engine Light or Malfunction Indicator Lamp (MIL), and set one of the following diagnostic trouble codes: P0115….Engine Coolant Temperature Circuit P0116….Engine Coolant Temperature Circuit Range/Performance P0117….Engine Coolant Temperature Circuit Low Input P0118….Engine Coolant Temperature Circuit High Input P0119….Engine Coolant Temperature Circuit Intermittent

On older pre-OBD II vehicles, the Check Engine light may come on if the coolant sensor is shorted, open or is reading out of range. GM coolant sensor codes include codes 14 & 15, Ford codes are 21, 51 & 81, and Chrysler codes are 17 & 22.

Coolant Temperature Sensor Replacement Cost

The average coolant temperature sensor replacement cost is between $50 and $250, depending on the car model and labor costs. A coolant temperature sensor costs $30 to $100, and the labor costs $20 to $150.

The coolant temperature sensor itself is often quite cheap and you can often find them around $40 for a quality one. There are cheaper ones on the market, but I heavily recommend buying a quality one like Bosch to skip the headaches.

The replacement is often also quite straightforward, except that you might have to pour out all coolant from the engine and refill it – which means that you have to remove all air from the coolant system, which can be difficult.

However, if you are fast replacing the sensor, there is often no need to tap out the coolant, but this requires some skills.

Remember to always make sure the coolant temperature is low when doing this kind of work!!

How to Spot a Faulty Coolant Temperature Sensor

Like any component under the bonnet, coolant temperature sensors can develop faults over time. A faulty sensor can lead to a range of problems developing, including overheating and poor engine performance. That’s why it’s important to know how to spot the signs of a faulty or failing temperature sensor, before it can cause further problems which could prove more expensive to fix.

Start by having a look at the unit itself to check its condition, as sensors/gaskets/connectors can develop cracks with extended use and continual temperature cycling. The CTS is usually found at the front of the car, near the thermostat housing, or on the radiator. While a visual check can help to diagnose some faults, not all problems with a CTS show visible symptoms.

Below, we list the other signs and symptoms which could indicate a CTS issue:

  • Irregular reading of the dashboard gauge (should be 88-90°C when the engine is warmed up)
  • Overheating engine (highlighted by dashboard gauge)
  • Check Engine Light alert on dashboard
  • Rough engine sound while idling
  • Limited performance (caused by ECU miscalculating fuel rich mixture)
  • Poor fuel economy

If in any doubt about which component is faulty beneath the bonnet, take your car to a professional mechanic for a complete diagnosis.

Diagnosing a Malfunctioning Coolant Temperature Sensor

Diagnosing should be the job of a certified technician. However, this shouldn’t stop you from examining the engine component if you suspect it is malfunctioning.

You may have to get your hands on a repair manual for the model of your vehicle. This will enable you to find the measurement values that a functioning coolant temperature switch should exhibit.

Check the service manual of your vehicle to locate your coolant temperature sensor. When you set eyes on it, disconnect the connector plugs.

If your sensor has two pins, determine the ohm measure between the pins. Then compare the value with the correct ohm-value – at a given temperature – in your repair manual. If the value does not tally, it means you need to replace the coolant temperature sensor.

If the values add up or tally, consider checking the connectors and wirings between the sensor and the engine’s control module.

You can also make use of an OBD2 Scanner to look for related trouble codes. Check the live data to see the temperature of the sensor. If the temperature is off the range, check the wirings of the sensor or replace the engine component.

What Happens If You Unplug The Coolant Temp Sensor?

Unplugging the engine coolant sensor while the car is running will likely cause the engine to stumble and run rough. The engine light may not come on imminently, but a DTC (Diagnostic Trouble Code) will be logged in the PCM.

You may also notice the engine fan turn on and off intermittently. The engine will run rich, so you’ll smell raw gas, and you may see black smoke from the tailpipe.

Unplugging the engine coolant temperature sensor before starting the engine may result in a no-start, especially likely on a cold morning. Although the PCM will see the sensor is unplugged and has a default fueling strategy for such an event. It is more of a one-size-fits-all approach, and so a cold start in colder temperatures may prevent the engine from starting.

COOLANT SENSOR VOLTAGE CHECKS

You can also use a voltmeter or digital storage oscilloscope (DSO) to check the sensor's output. Specs vary, but generally a cold coolant sensor will read somewhere around 3 volts. As the engine warms up and reaches operating temperature, the voltage drop should gradually decrease down to about 1.2 to 0.5 volts. If you're using a scope to display the voltage signal, you should get a trace that gradually slopes from 3 volts down to 1.2 to 0.5 volts in three to five minutes (or however long it normally takes the engine to reach normal operating temperature).

If the voltage drop across the coolant sensor reads at or near 5 volts, it means the sensor is open or it has lost its ground connection. If the voltage is close to zero, the sensor is shorted or it has lost its reference voltage.

When working on older 1980s vintage Chrysler products, watch out for a sudden voltage increase as the engine warms up. This is normal and is produced by a 1000 ohm resistor that switches into the coolant sensor circuit when the sensor's voltage drops to about 1.25 volts. This causes the voltage to jump back up to about 3.7 volts, where it again continues to drop until it reaches a fully warmed up value of about 2.0 volts.

Sometimes a coolant sensor will suddenly go open or short when it reaches a certain temperature. If your voltmeter has a "minimum/maximum" function, you can catch sudden voltage fluctuations while the sensor is warming up. If you are viewing the voltage pattern on a scope, a short will appear as a sudden drop or dip in the trace to zero volts. An open would make the trace jump up to the VRef voltage line (5 volts).

If the coolant sensor reads normally when cold (high resistance and 3 or more volts), but never seems to reach normal temperature it could be telling the truth! An open thermostat or the wrong thermostat may be preventing the coolant from reaching its normal operating temperature.

Replacing the Car Temperature Sensor

The sensor will eventually need to be replaced altogether after time. If the engine sustains any kind of trauma or damage, sensor replacement is always recommended because you don’t want to risk running the vehicle with a faulty one. Even general wear and tear can cause the sensor to erode over time.

You can always have your CTS replaced by an auto care professional. This aspect of preventative maintenance can certainly save you some headaches and hassle in the long run.

Hint – if you can’t find a temperature sensor in your car – this article should help.

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