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Water temperature light: what to do if it is on or flashing?


What is the red coolant sight glass?

The red coolant sight glass is represented by a thermometer immersed in a liquid, symbolized as a wave.

The more imaginative may also describe the water temperature warning light as a "sailboat in the sea" or a "buoy in the waves". Whatever you call it, it's important to know what it means and what to do when it's on or flashing.

Water temperature warning light and coolant sight glass?

We often hear about water temperature sight glass and coolant sight glass and many people wonder what the difference is between the two.


There is actually no difference between these two warning lights. They indicate exactly the same thing, namely the temperature in degrees Celsius of the fluid responsible for cooling the engine. It is no coincidence that coolant is often referred to as"coolant".

In the early years of motorsport this fluid was simply ordinary tap water. However, over the years various fluid mixtures have been developed that are increasingly efficient in their ability to remove heat.

The most common of these coolant mixtures is water and glycol, almost always mixed in a 1:1 ratio (1 litre of water and 1 litre of glycol). The advantages of this mixture are mainly the decrease of the freezing temperature and the increase of the boiling temperature.

With the lowering of the freezing temperature, the water-glycol coolant remains liquid up to -40° compared to only 0° for water. In this way, a car parked outdoors in winter will have no problems starting and running. In some Nordic countries where the cold reigns for several months of the year, the practice was in fact to mix animal fat with diesel fuel to prevent freezing.


Instead, increasing the boiling temperature of the coolant from 100° to 135° ensures greater heat removal even in the hottest months or with broken engines.

These technological improvements have certainly increased the performance of the coolant. However, we must always remember that when the water temperature warning light is on, the only thing to do is to stop the car immediately and look for the problem. A running engine that is not being cooled effectively will melt in a short time causing thousands of euros of damage. Alerted man...

How does the water temperature warning light work?

The red coolant warning light is almost always on either inside the corresponding temperature gauge or in the center of the dash in the digital area. The temperature gauge has a lever that indicates the temperature up to 130°C. However, the temperature should not exceed 100°C under normal driving conditions.

The coolant temperature gauge is very similar to the oil temperature gauge with which it is often confused. It goes without saying that the oil gauge shows the typical oil sight glass with the drop and the sight glass is usually yellow.

What is the normal coolant temperature of the car's engine?

Although the coolant temperature gauge reaches 130°C, the temperature must be well below this threshold to be considered acceptable.

The temperature of the coolant under normal driving conditions is between 80 and 90°C. However, it can rise after a long climb where the engine has been running a lot or if the engine has been running for a long time. For the coolant recirculation system to work optimally, the engine must be in full operation. If we have been stopped for a long time at very low revs it can be normal to notice an abnormal temperature.


If we see that the water or coolant temperature often exceeds 100°C, there could be problems and it would be advisable to have the car checked.

When the water temperature light comes on: causes...

There are many causes for the engine water temperature warning light to come on and they are usually due to a problem in the cooling system:

  • Prolonged stalling with the engine running - if you keep the engine running for a long time without moving the car, the radiator may not be able to cool the fluid. This is because the radiator needs cool air generated by the movement of the car to effectively cool the fluid. If the car is not moving, the air is not cool enough and the radiator will not be able to cool the fluid. This causes the water temperature warning light to come on. Simple!
  • Uphill road andloaded car - if we start running uphill with a heavy backpack, even the most sporty will start to work up a sweat and get hot. The same goes for the car where the engine struggles, the radiator doesn't cool down fast enough and the fluid gets hot, causing the red water temperature light to come on.
  • If we are driving in summer and the outside temperature is close to 40°C we are not the only ones affected. We can turn on the air conditioning and cool the cabin, the car unfortunately cannot. Therefore, the car will struggle to get fresh air through the radiator to cool the liquid. The situation is further aggravated if you drive in the city with constant braking at traffic lights and intersections and if you consume a lot of electricity, for example with the air conditioning. So don't be surprised if the water temperature warning light comes on in these conditions.
  • Low coolantlevel - if the coolant or water level falls below the safe level, the warning light will come on to draw our attention and prevent engine damage. Checking the coolant level and topping up the coolant should be done regularly to avoid being in the middle of nowhere with the water warning light on. Remember that in a major emergency it is possible to use distilled water to top up and get to a petrol station or workshop safely.
  • Mechanical damage to a coolingsystem component - this is the worst possible cause of the coolant warning light coming on. If we've already checked that the coolant level is good and we've left the car idling for a few hours but the warning light is still on, then there is probably damage to a component. Components that could be affected could be the radiator, water pump or service belt. However, if these components are intact, the problem could be a dry and damaged gasket, typical of cars that are several years old. Rubber, being a natural material, tends to dry out, weakening and creating cracks and fissures. The only way to verify the exact cause is to immediately rely on a good mechanic.

What to do with the water temperature light on?

If you see the red water temperature light on, the first thing to do is to stop safely and turn off the engine. Wait 15 minutes and try to restart the engine. If it doesn't start again, it means that the cause was just a faulty electrical contact or fuse, a steep rise or that the outside temperature was too high.


However, if the water indicator light remains on, avoid restarting the engine and immediately check that there is no water vapor under the hood. This would, in fact, be irrefutable proof of a water leak and would make it necessary to call a tow truck.

Let's then check that the coolant level is sufficient (do this with the engine off and cold to avoid splashing boiling liquid). If the level is low, we fill it up (in case of emergency, mineral water is also fine if we don't have the coolant at hand). Once restarted, the warning light should go out, no problem then.

If the sight glass is still on even though the fluid has been topped up, the problem is a damaged component. We do not recommend that you drive again, as this could seriously damage the engine and create thousands of euros of damage that needs to be repaired.

The best thing to do is to call a tow truck and take it immediately to the nearest garage where the damage will be repaired. Believe me the cost of calling a tow truck is nothing compared to the damage caused by an overheated engine.

What to do about the flashing water temperature light?

If your car's water temperature light starts flashing it means the engine is overheating. Some car models have a flashing water temperature light while others have a steady light. Other models add a sound indicator to further attract the driver's attention.

Any sign that appears on the dashboard about the water or coolant temperature should never be ignored.

The procedure to follow with the water temperature warning light flashing is the same as described above for the water temperature warning light on. The important thing is to always stop safely, turn off the engine and wait a few minutes.

What is engine coolant?

Engine coolant is a compound of water and ethylene glycol (in chemistry C2H6O2, also known as antifreeze). Its task is to absorb the heat generated by the car engine and transport it to another location, disperse it and restart.

The liquid is in fact inserted into a circuit called "cooling" which involves several components. One of the most important is the radiator which has the task of exploiting the flow of cold air generated by the movement of the car to cool the hot coolant. Under normal conditions, the flow of fluid keeps the engine at the proper temperature and prevents the water temperature warning light from coming on.

A damaged component or leak anywhere in the circuit can cause the coolant level to drop, potentially causing the engine to melt. To prevent such serious damage, all cars are equipped with a coolant level indicator/warning light and a temperature warning light. Normally, when the coolant level drops and the temperature rises, you can be sure that there is a leak.

Why does the car's water warning light often come on?

The car water warning light should not come on unless there is an ongoing problem with the engine cooling system. Therefore, any "abnormal" ignition should not be underestimated and should be treated as a possible fatal problem for the car.

However, if we are 100% sure that there are no problems with the engine cooling system, perhaps because we have just had the work done by a trusted mechanic, then the ignition of the water light could be caused by a small problem with the electrical system or control unit.

The water temperature light of modern cars is in fact an electrical sensor that constantly sends temperature data to the control unit. It is enough for the sensor, the connection cables or the control unit itself to be faulty to cause abnormal ignitions.

Normally, however, such a problem causes several lights to come on and it is very rare that only the water indicator light comes on. Therefore, the best advice is to rely on an electrician to check the condition of the electrical system and understand where the problem lies.

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