Can a bad coolant temperature sensor cause car not to start?

Modern cars nowadays act according to the readings of various sensors. Most of these devices analyze some parameters and transmit information to the electronic control unit (ECU).

To give an answer, the coolant temperature sensor will not cause the car not to start, although one of its bad symptoms is engine stalling soon after starting, especially when the engine is cold. But if your car does not start, consider checking for other issues first.

If there are signs of a malfunction regarding the coolant temperature sensor, then this sensor should be replaced. Incorrect information can cause serious malfunctions in the operation of the engine and you will see these signs as soon as you start the car and, also, other faults.

The system must have a normal coolant level for the coolant temperature sensor to be accurate. Therefore, you need to add coolant to the system in time.

What does the coolant temperature sensor do?

When the engine is running, heat is generated from the cylinder block. The coolant is responsible for removing the excess heat, circulating through the unit’s system, and the radiator. When analyzing what the coolant temperature sensor affects, you need to understand how this device works. Indeed, its task is to send signals to the ECU about the engine’s temperature.

Influencing the performance of this sensor, it is possible to increase the controllability of the car when driving on an unheated engine, stabilize the idle speed, and reduce the level of harmful emissions. A bad coolant temperature sensor or its transmission of distorted data to the control unit will create noticeable trouble.

Thanks to signals from coolant temperature sensor, the below process will work just fine.

  1. Fuel enrichment process. If the ECU receives information about the low temperature of the coolant, then the injection time for the injectors is recalculated. This action contributes to the stability of idling. Gradually, the temperature rises, and, based on these readings, the injectors deplete the mixture. If the sensor does not give correct information, then a rich mixture will occur
  2. Increase in rpm during start. The engine may stall if the engine speed is insufficient when starting. A command from the ECU to accelerate the engine speed helps to get rid of this so that the car does not stall.
  3. Exhaust recirculation. To maintain controllability during startup, the recirculation valve must be closed before the system reaches operating temperature. If this does not happen, then the car will get unstable rpm or a stalled car.
  4. The torque converter clutch in the gearbox is not blocked until the engine warms up. This is done to maintain optimal handling.
  5. Turn on the cooling fan. Based on data from the coolant temperature sensor, the radiator fan starts or turns off. It helps to reduce the temperature of the refrigerant faster. In some car models, a separate sensor with a single function is used exclusively to start this fan.

Where is the coolant temperature located?

The sensor is located, as a rule, in the intake manifold not far from the thermostat. Less commonly, manufacturers install this sensor closer to the cylinder head. In V-shaped engines, a pair of these sensors are mounted on each row of combustion chambers. 

A pair of sensors can also be installed separately for the fan and ECU.

coolant temperature sensor location

Symptoms of bad coolant temperature sensor

Although the coolant sensor is not directly responsible for a car that does not start, you should check for these signs of malfunction regarding this sensor.

Rough cold start

The engine starts and immediately stalls. After warming up, this problem goes away. This happens because the coolant temperature sensor may give incorrect readings to the ECU. For example, that the engine is already warmed up to the optimal temperature. 

To start a cold engine, you need more fuel than a hot one. And since the ECU “thinks” that the engine is hot, it doesn’t provide the engine enough fuel.

Bad hot start

Here everything is exactly the opposite. The coolant temperature sensor can give wrong readings to ECU that the engine is cold. As a result, the engine will simply fill up with fuel. Moreover, the P0172 code may appear in this situation.

Increased fuel consumption

This is a consequence that follows from the symptom above. If the engine is flooded with fuel, then the consumption will increase.

The cooling fan turns on or off for no reason

Because the coolant temperature sensor is giving the wrong readings, this will cause the radiator fan to turn on when it’s not needed, for example, the engine has the correct working temperatures and the sensor reads higher temperatures, which will logically trigger the cooling fan to turn on.

Things worsen if the opposite happens, the sensor can sometimes underestimate the readings. The coolant temperature may already reach the boiling point, and the sensor will think that the temperature is normal, and therefore the ECU will not turn on the cooling fan.

Check engine light on

In most cases, you will see the check engine light error appear in the car’s dashboard, and sometimes the coolant temperature error light because of the incorrect readings of the sensor.

How to check the coolant temperature sensor?

The easiest and fastest way to check the coolant temperature sensor is to simply remove it from the slot. You’ll imediatelly see that the engine will go into emergency mode, the cooling fan will turn on, the fuel mixture will be prepared based on the readings of other sensors. If, at the same time, the engine began to work better, then the sensor definitely needs to be replaced.

The next test of the coolant temperature sensor requires diagnostic equipment such as an OBD. First, you need to check the temperature readings on a cold engine, for example, in the morning. The reading should correspond to the ambient temperature. 

A small error of 3-4 degrees is allowed. And after starting the engine, the temperature should rise smoothly without jumping between readings. If the temperature was 30 degrees Celsius or 86 °F, and then sharply became 32 °C or 33 °C (90 °F or 92 °F), this indicates a sensor malfunction.

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