Can You Drive With A Bad Coolant Sensor? (Solved!)


Car trouble can cause a ton of anxiety, breakdowns, broken schedules, the possibility of a large workshop bill…all set off your internal alarm. In this post I’ll restore calm by answering all your likely questions.

Can you drive with a bad coolant sensor? It is possible to drive a vehicle with a faulty coolant temperature sensor as the management system defaults to a static reading. A vehicles coolant sensor is a critical component used by the engine management system. It directly effects, cooling and fueling of the engine and therefore effects how the engine performs.

By the end of this post you’ll know how the sensor works, how to diagnose a faulty sensor, wiring and control module. You’ll also know the simple steps involved in replacing the sensor.

What Does Temperature Coolant Sensor Do?

Your vehicles engine coolant temperature sensor (ECT) also known as coolant temperature sensor (CTS) is a small but important piece of kit. The sensor itself isn’t very impressive, made from brass to prevent corrosion it can fit easily in the palm of your hand. The information it supplies however is critical to how your vehicle runs.

The sensor, fitted to the engine close to the thermostat housing supplies the engine management system with real time engine coolant temperature readings. The PCM (Power-train Control Module) relies on this information to make various important decisions about fueling, cooling, power application and emissions. In turn, other on-board control modules such as HVAC for example will make decisions based on these temperature readings also.

The little temp sensor is pivotal to how your motor runs, you could say it fights way above its weight.

How Does Coolant Sensor Work?

The sensor comprises of an electrical connector end and a probe end. The sensor simply threads into the engine with the probe end submerged in coolant inside the engine.

The sensor operates by changing resistance proportional to how hot it’s probe end is. A typical sensor will when cold read about 10,000 ohms and drop to about 200 when the engine is up to operating temperature. Don’t sweat these details I’ll show you how to test ohms later, it’s easy!

The two wire electrical connector fitted to the sensor comes from the PCM. One wire supplies the sensor with 5 volts, this is known as reference voltage. The second wire carries the reduced signal voltage back to the PCM. And since resistance is like a blockage in a garden hose pipe, not all the voltage returns, it’s this reduced voltage the PCM uses to determine the coolant temperature.

The higher the coolant temperature the less resistance the sensor offers and that translates into a higher return signal voltage to the PCM.

It’s worth noting not all sensors work in this way. This type sensor is however the more common type and is known as a negative temperature coefficient thermistor. A positive temperature coefficient is inverted, as the coolant temperature rises the resistance increases.

Some sensors may be a three wire set up, don’t let this throw you off. The third wire is usually employed to feed the dash temperature gauge.

And some higher end models may employ more than one ECT, a V engine may have an ECT for each bank. Some systems use two ECT’s one for the PCM and a second for the radiator fan.

Can Bad Coolant Temperature Sensor Cause Car To Stall?

ECU

Symptoms of a bad sensor vary, but a check engine light will be the common denominator. It’s such an important sensor, an engine light will alert the driver as soon as the on-board 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

How To Test A Coolant Sensor

If your car won’t start from cold try applying a little gas pedal, this signals the PCM to override and add some gas. If the engine now starts, you likely found the problem.

My more usually diagnoses starts with a visual. Checking for simple easy wins. In the case of a faulty ECT, I’ll check things like:

  • Coolant level, insufficient coolant will cause the sensor to operate erratically if at all.
  • I’ll look for loose wiring connector.
  • Coolant sensor wiring damage like chafing or rodent damaged.
  • Coolant condition, bad coolant can coat the sensor causing a bad read.

With all the low hanging fruit checked, I’ll turn my attention to the DTC’s. Modern cars as you know record all major faults in the PCM. Makes sense to start your diagnoses from there, it will of course mean you’ll need to borrow or buy a code reader.

Scan tool

Common ECT fault codes are:

  • P0115 – Circuit fault
  • P0116 – Circuit range
  • P0117 – Low volt
  • P0118 – High volt
  • P0119 – Intermittent fault

Testing a sensor, PCM and it’s circuit will require a scan tool that reads live data. These type tools are a step above a simple code reader but still aren’t very expensive and will likely pay for them selves many times over.

Here’s a basic tests you can perform to help diagnose a faulty ECT sensor. Bear in mind, sensors aren’t that expensive and although I’m not in the habit of throwing parts at a car I do understand, doesn’t make sense buying test equipment to diagnose a $20 part. But for those that are interested, buckle up!

Testing ECT resistance

This test can be performed in or out of the vehicle, however, the results won’t be conclusive. A good scan tool has the ability to graph ECT performance and makes it easy to see interference or a signal dropping out momentarily. An intermittent fault will be difficult to catch using a voltmeter. But this test will work great for a dead sensor.

ECT test

Go ahead and remove the sensor plug, using a DVOM set to ohm’s. Check resistance when the engine is cold and note the reading. Plug the sensor back in, start the engine an allow idle for a few minutes.

Shut the vehicle off and repeat the resistance test, a working negative coefficient sensor will read a smaller resistance value. If your reading is unchanged, your sensor is dead. A lower resistance reading doesn’t of course mean the sensor isn’t faulty. Only graphing the senor through a full heat cycle

ECT sensor test

Your vehicles sensor will have a specified resistance range for a specified temperature. It’s possible to test the sensor successfully using a thermometer, a spec chart, DVOM and hot water.

ECT testing

Testing ECT volt drop

As you know the coolant sensor receives a 5v reference supply from the PCM. The second wire is the signal wire and is used to carry the reduced voltage back to the PCM. The resistance in the sensor drops as the engine heats.

ECT testing
ECT volt test

Measuring the volt drop across the sensor allows us observe the sensor work in real time. The engine will need to be running and the test will need to be performed through the heat cycle. Voltage differential reduces as the engine heats (resistance drops).

ECT reference voltage check

This test is a straight forward reference voltage check. We’ll need to disconnect the sensor, with ignition on. Back probing is the best approach, using excessive force on the contact points can cause them to widen. 5 volts or close to it means all is OK here.

ECT ref volt test

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 tail pipe.

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.

How To Replace A Coolant Temperature Sensor

Replacing the sensor is not a difficult job, but you will need a few simple tools such as:

  • Drain pan
  • Small flat screwdriver
  • Deep socket and ratchet
  • Fresh coolant.
  • Clean cloths
ECT removal
ECT fitting

The sensor as you know is located close to the thermostat housing on most models and the procedure is as follows:

  • Place a drain pan under the engine, some coolant will flow from the ECT housing. Have the replacement ready to install and you’ll keep the mess to a minimum.
  • Use the flat screwdriver (if needed) to remove the electrical plug connector. Set aside, don’t allow coolant to get on the contact pins.
  • Using the deep socket and ratchet, remove the sensor.
  • Fit the new sensor ad washer (if applicable) by hand first before tightening. This isn’t a lug nut and it’s brass, so please be gentle. Most are torqued to about 15 lb ft, but check your model specs. The sensor should come with thread sealer, if not use some pipe dope.
  • Refit your electrical connector.
  • Top up your coolant.
  • Run the vehicle and check for heat from the heater. If you lost a ton of coolant, you’ll need to bleed the system. You run a risk of damaging the engine if the system is air locked. I wrote a post about that very procedure and you can check that out in this post “No heat at idle”.
Top up coolant

Related Question

How much does it cost to replace a coolant temperature sensor? The cost of a coolant temperature sensor ranges from $20-$50. A garage may charge $100-$250 to fit a coolant sensor. Fitting the sensor will usually include a replacing the coolant and purging trapped air from the coolant system.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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