Engine temperature warning light - designed to inform the driver about a dangerous overheating of the cooling system. It is usually located next to an arrow or digital temperature indicator. Refers to emergency indicators, upon activation of which further operation of the vehicle is prohibited. The pictogram of the overheating lamp has the form of a thermometer with two waves. The activation of the temperature indicator indicates that the engine cooling system is not coping with its functions.

The temperature light is on - what to do?

If the overheating lamp flashes or stays on, stop the car as soon as possible and turn off the engine. You can start it only after a while. This may be necessary in order to find out the reason for the operation of the indicator and try to fix the breakdown on the way. If the fault cannot be determined, the vehicle must be towed or towed to your home or service station.

Dangerous delusion. Practice shows that some drivers have a strong opinion - do not turn off the engine when the overheating lamp is triggered. Allegedly, this will lead to irreparable consequences. The engine can indeed be cooled more efficiently when it is running. But only on condition that the speed is sufficient to create a powerful flow of oncoming air and the radiator fan is working properly. But relying on these factors is dangerous. It is much safer and more correct to immediately stop the car, observing the rules of the road, and turn off the engine.

Attention! Use extreme caution when opening the hood to determine the cause of the engine overheating. There may be accumulated steam under it, which will cause serious burns to the hands and face. In case of depressurization of an overheated cooling system, steam is usually visible even with the hood closed. This is a signal for caution.

The overheating lamp is on - what can not be done?

In the event of an engine overheating alarm, it is strictly prohibited:

  1. Keep moving, hoping for luck.
  2. Douse the engine with cold water to speed up its cooling.
  3. Cover the underhood area with snow.
  4. Add plenty of cold water to a hot system.
  5. Use a fire extinguisher if you suspect a fire.

A sign of a fire in the engine compartment is black smoke. White thick steam indicates a depressurization of the cooling system.

The importance of operating temperature for an engine

The operating temperature of an engine is the temperature range between about 60 and 95 degrees Celsius at which the engine will perform at its most efficient. If the coolant is colder or hotter, power drops, fuel consumption increases, and wear of parts accelerates. Overheating is especially dangerous, which can cause depressurization of the cooling system and destruction of engine elements.

If the problem is ignored, the engine can seize in minutes due to excessive thermal expansion. For the same reason, the cylinder block may crack and the head may be deformed. Very often, repairing an overheated engine is impractical and it has to be completely replaced. Therefore, it is very important to replace the overheating alarm in a timely manner, stop the machine, turn off the engine, and do not operate it until the cause of overheating is found and eliminated.

Wiring diagram and modes of operation of the engine temperature light

The critical engine overheating alarm system consists of only two elements - a warning light and a temperature sensor.

The overheat warning lamp is located on the dashboard in front of the driver's eyes. A prominent place is usually chosen for it, next to the arrow or digital temperature gauge. When the cooling system copes with its functions, the overheating lamp does not light up. It may briefly flash when the ignition is turned on, which allows you to verify that the system is working. The main task of this indicator is to operate in case of dangerous overheating of the engine.

The connection diagram for the temperature light is shown in the figure below. The principle of operation of the alarm system is very simple. The temperature sensor is a normally open sensor, the contacts of which close the system to ground when a dangerous temperature is reached. "Plus" power is supplied to the lamp with the battery through the fuse and the ignition switch. As long as the coolant temperature is below the sensor threshold, the lamp does not light. When the temperature exceeds a dangerous one, the contacts in the sensor and the entire circuit are closed. The warning lamp is powered by the battery and comes on to alert the driver that the engine is overheating.

Wiring diagram for engine temperature lamp
Wiring diagram for engine temperature lamp

10 reasons why the engine overheat indicator is on

Within the framework of this material, those reasons for the operation of the engine overheating lamp that are most often encountered in practice are considered:

  1. Low coolant level.
  2. The thermostat does not open.
  3. The thermostat opens with a delay.
  4. Dirty radiator.
  5. The radiator fan has failed.
  6. The fan switch does not work.
  7. Water pump failure.
  8. Malfunctions in the engine lubrication system.
  9. Overload loads.
  10. Other reasons.

Below is a summary table that lists all the main causes of the engine overheating alarm and the actions to be taken in a particular situation.


Reasons for turning on the overheating lamp and correct actions



Low antifreeze

Check the coolant level, add antifreeze if necessary, determine the cause of the leak

Thermostat stuck

Check thermostat, replace if necessary

Thermostat opens late

Check the thermostat in boiling water, if the temperature does not match, replace

Dirty radiator

Inspect the radiator cells, in case of severe contamination - clean using special products

Fan not working

Make sure that the fan is really not working, check the operation by connecting it directly to the battery

Failure of the sensor of inclusion of the fan

Check the fan by connecting to the battery directly, if it works, leave it like that until the house or service station, where to replace the faulty sensor

Broken cooling pump

Check pump, replace if necessary

Lubrication system malfunctions

Check the lubrication system, determine and eliminate the cause of overheating

Excessive loads

Reduce the load on the engine, take a break so that the cooling system cools down

Other reasons

Determine and eliminate the cause of motor overheating from the list below


Further, these reasons are considered in more detail - what is the essence of the malfunction, how to diagnose and eliminate it. In some paragraphs there are links to detailed materials on the topic, which are recommended to read.

Low coolant level

Insufficient level of antifreeze in the cooling system inevitably leads to engine overheating. The first step is to check the coolant level. To do this, two marks are applied to the expansion tank of the cooling system - a minimum and a maximum. If the fluid level is below the minimum mark, it is necessary to add antifreeze or water to the system. It should be remembered that the engine cooling system is a sealed system, and the working fluid should not leave it. Accordingly, a low level of antifreeze indicates the presence of a leak that must be found and repaired.

Antifreeze level below normal
Antifreeze level below normal

Attention! Water can be added to the cooling system only in the warm season. In winter, such actions can lead to freezing of the liquid, which will cause the destruction of the cooling system. Regardless of the time of year, after adding water to the system, it is recommended to completely replace the coolant. It is advisable to do this after eliminating the leak and the cause of overheating.

Thermostat does not open

The thermostat is a valve that separates the small and large circuits of the cooling system. As the engine temperature rises, the thermostat gradually opens. Hot antifreeze is bled into the radiator, where it is cooled by an oncoming air flow or a fan, after which it is returned back to the small circuit.

If the thermostat is stuck in the closed position, superheated coolant will not enter the radiator. Because of this, it overheats very quickly and the temperature light on the dashboard comes on. A link to material on a complete thermostat test is provided in the next paragraph. A stuck thermostat should be replaced with a new one.

The thermostat opens with a delay

The valve is structurally designed to open at a certain temperature of the coolant. If it is worn, defective, or incorrectly fitted, it may open too late. This may well be the cause of the engine overheating and the temperature light on the dashboard.


There is a separate detailed material about the thermostat, its typical malfunctions and diagnostics on the Auto without a service station website. You can get to know him at this link.

Dirty radiator

The main radiator is an element of a large circuit of the engine cooling system. It is intended for utilization of heat taken away by a small circuit. The antifreeze in the radiator is cooled in two ways. Naturally - with an oncoming incoming air flow during the fast movement of the car. When the speed of the machine is insufficient or it stands still, the fan is responsible for blowing the radiator.

If the radiator cells are contaminated with dust, insects, leaves and other debris, the antifreeze will return to the small circuit insufficiently cooled. This can cause the engine to overheat and cause the coolant temperature light to come on.

Dirty radiator
Dirty radiator

To establish this cause, it is necessary to inspect the radiator. If its cells are contaminated, the part must be cleaned and washed using special products.

Failed radiator fan

Radiator fan - designed for forced cooling when the oncoming air flow is not enough. It turns on when a certain temperature is reached by means of a sensor installed directly in the radiator. Some vehicles have two fans that run in series.

Dual radiator fan
Dual radiator fan

To make sure that the fan works in the field, it can be connected directly to the battery using two wires. If the fan works, but does not turn on when the temperature lamp is active, the switch-on sensor is most likely out of order.

Fan switch not working

The fan switch sensor is located directly on the radiator and is a normally open sensor. When the antifreeze temperature reaches a certain point, the contacts in the sensor close and power is supplied to the fan. If the machine has two fans, then each of them is turned on by its own sensor, designed for a certain temperature.

Usually one fan is turned on first. If his work is not enough to cool the radiator, a second fan turns on to help him.

Fan ON sensor
Fan ON sensor

To test the fan switch in the field, you need to verify that the fan is working. To do this, it must be connected directly to the battery. Often there are not long enough wires in the trunk to perform this operation. In such cases, another method can be applied. To do this, you need a paper clip or a small piece of wire. With its help, you need to short-circuit the contacts of the fan switch-on sensor, thereby excluding it from the circuit. The fan will start to rotate constantly, which will allow you to safely get to the house or service station.

Water pump failure

The cooling system pump is a pump that provides a constant circulation of antifreeze through the engine cooling system. It is driven by the crankshaft through a belt. If the pump fails, there is no coolant circulation. Overheated antifreeze is not dumped into the radiator for cooling. The engine overheats almost instantly, which triggers the temperature control light on the dashboard.

Water pump
Water pump

In the field, the cooling system pump is almost impossible to check. The only thing that can be done is to make sure that the belt through which the pump is driven is in good condition. More detailed diagnostics and repairs should be carried out in a garage or at a specialized service.

Malfunctions in the engine lubrication system

Engine lubrication system - designed for lubrication and partial cooling of rubbing parts. In the event of a malfunction, it can cause a critical overheating of the engine, which the cooling system is unable to cope with. The main malfunctions of the lubrication system are insufficient oil level and low pressure.

To check the oil level in the crankcase, you need to use a dipstick specially designed for this. Some vehicles have an oil level indicator light located on the dashboard.

Simultaneous operation of oil pressure and engine overheating lamps
Simultaneous operation of oil pressure and engine overheating lamps

An oil pressure warning lamp indicates insufficient pressure in the engine lubrication system. Unlike the level sensor, this indicator is on all cars. It is made in the form of a red lamp with the image of an oil can. Lights up when the ignition is turned on and goes out after the engine has been successfully started. There is a separate detailed material about the oil pressure lamp on the Auto website without a service station.

Extreme loads

The cooling system of a car is designed to operate in a wide range of loads. However, there are loads that cause the engine to overheat, which the cooling system cannot cope with.

The coolant temperature lamp may flash briefly or stay on when the machine is used for a long time in the following modes:

  • a long climb up a mountain or driving along a serpentine at low speed;
  • driving with an overloaded trailer;
  • towing another car;
  • driving on a bad road or off-road;
  • aggressive driving;
  • traffic jams in hot weather.

If the engine overheating lamp comes on in any of these driving modes, stop the vehicle and turn off the engine. After a while, the cooling system will cool down and you can continue driving.

Other reasons

Engine overheating can be caused by other factors not mentioned above:

  • bad car rolling;
  • driving with the handbrake on;
  • faulty chassis (wheel bearings, jammed brakes);
  • faulty air conditioner;
  • breakdown of automatic transmission;
  • abnormal tuning, due to which the main radiator is poorly ventilated;
  • abnormal heat;
  • poor quality or worn coolant.

Another fairly common reason for the inefficient operation of the cooling system is dirt and scale in the radiator and engine water jacket. Occurs when hard water or low-quality antifreeze is used as a working fluid. Eliminated by flushing the cooling system.

Checking the operation of the overheating alarm lamp

The overheat alarm system is vital to the engine, so it needs to be checked from time to time. On modern vehicles, a diagnostic mode is provided for this, which is briefly activated when the ignition is turned on. At this moment, you need to carefully look at the dashboard - if the temperature lamp previously lit up, but now it does not light up, then you should not start the engine until the breakdown is repaired.

If the diagnostic mode is not available on the machine, you can test the overheat alarm system as follows. First, you need to remove the lamp that is responsible for the indication and check it by connecting it directly to the battery or another 12 volt voltage source.

Secondly, you can remove the temperature sensor responsible for turning on the emergency lamp and check it by heating. A multimeter is connected to its outputs in the dialing mode. If the circuit closes when heated, then the sensor is working. In this case, it should be borne in mind that the sensor may not work in boiling water, since it is designed for a temperature that is higher than the boiling point of water. You can heat the sensor with a lighter or a gas burner.


The engine temperature light is an emergency indicator that informs the driver of a critical overheating of the cooling system. If activated, stop the vehicle and turn off the engine as soon as possible. Until the cause of the alarm is clarified and eliminated, further operation of the machine is strictly prohibited. There is a separate material about engine overheating, its signs and possible causes on the Auto without Service Station website.