BMW Coolant Light Comes On and Off – Causes and Solutions

BMW coolant light comes on and off only when there is a problem with BMW vehicles.

BMW Coolant Light

In this guide, you will learn why your BMW car coolant’s light may come on and off and how to fix the issue. Read on to learn all you need to know about coolant light problems.

Reasons Why Your BMW Coolant Light Goes On and Off

The common reasons your BMW’s coolant light goes on and off are a defective coolant sensor, inaccurate antifreeze used for the vehicle, and leakage in the automobile’s cooling system. Other possible reasons include low coolant levels and extreme temperatures outside the internal combustion engine.

  • Defective Coolant Sensor

A coolant temperature sensor measures the coolant’s temperature and indicates how much heat the engine is giving off. The coolant level sensor operates with the engine control unit (ECU) and continuously observes the coolant temperature to ensure the engine works at the optimum temperature.

One of the common reasons why your coolant’s light might come on and off could be due to a faulty sensor. A defective coolant sensor will make the coolant’s light come on despite the coolant still being full. Once the sensor is bad, it won’t send the correct signal to the vehicle’s computer system.

You can easily confirm if the sensor is the reason why the coolant’s light is turning on by checking to see if the water bottle is full or not. Moreover, you should know that coolant sensors don’t last forever and will wear out with time. So, once the BMW has covered considerable mileage, ensure you inspect the coolant sensor.

Other symptoms of a bad coolant sensor include illuminated check engine light, black smoke from the exhaust, difficulty in starting the car, rough idling, radiator fan problems, and transmission shifting problems.

  • Use of Incorrect Antifreeze

Antifreeze, also known as engine coolant, is a colored liquid mixed with water to regulate the engine during extreme temperatures. Once the temperature outside the internal combustion engine switches from hot to cold, the coolant is pumped throughout the engine block to maintain the optimum temperature.


The antifreeze prevents the water inside the engine’s cooling system from freezing during winter or extreme cold temperatures. This is achieved by lowering the freezing point of water.

Although every coolant comprises the base chemical ethylene glycol, they all differ because of the additives and dyes used in their production. BMWs are known to use specific antifreeze brands as their engine’s coolant. Many car owners make the mistake of using varying brands of antifreeze, which ends up affecting the vehicle’s engine or making the sensor defective.

Reasons of Faulty Light on BMW

Any coolant used for a BMW car must have a low freezing point, high boiling point, and a specific ratio of water and antifreeze. If an incorrect coolant is applied to the system, the sensor will start to malfunction, signified by the coolant’s light turning on and off. It can likely escalate to a bigger issue, where the cooling system is damaged, and the engine starts to overheat.

  • Leakage in the Cooling System

The coolant’s light coming on and going off can be due to a leak in the BMW’s cooling system. The cooling system of a BMW is made up of varying components, including a coolant radiator, thermostat, coolant pump (mechanical or electric), expansion tank, tubes, engine fan, and engine temperature control sensor.

The parts work together to ensure the engine maintains an even temperature for optimum performance. If any part of the system experiences failure, it can cause engine overheating, leading to more problems. The first place to check for signs of leakage is the expansion tank (water bottle).

Also, check the water pump, radiator, and pipes to see if they have been damaged or broken anywhere. Cracks in the plastic pipe connection at the bottom of the radiator can cause a leak in the BMW radiator. More so, a broken radiator seal can affect the pressure in the coolant system, causing its trigger light to turn on. Furthermore, a gasket leak may result in an exemption of coolant from the system.

  • Low Coolant Level

Coolant keeps the engine running at the optimum temperature and prevents overheating and freezing. The coolant operates with the aid of its sensor inside the expansion tank.
There is a required antifreeze level for the engine’s operation, and if the coolant is below this level, it can trigger the coolant warning light. If the coolant’s level goes down faster than usual, it could be due to leaks and cracks in the cooling system.

  • Extreme Temperatures

Driving a BMW car in cold or freezing temperatures can cause the coolant fluid to contract. The same applies to parking the vehicle in extremely cold temperatures. The contraction of the coolant can cause the coolant sensor to light up. This phenomenon often causes the antifreeze level to go down, and this will make the sensor warning light to be switched on.

How To Fix a BMW Car Coolant Light Coming On and Off

You can fix your BMW car coolant light that comes on and off by repairing the coolant sensor or replacing the sensor with a new one. Other fixes include replacing the coolant used or changing it and repairing the leakage in the automobile’s cooling system.

  • Repair the Sensor

Faulty connections to the sensor can cause the coolant sensor to become bad or defective. Similarly, corrosion on sensor terminals, low coolant levels, and air pockets also result in a faulty coolant sensor. Immediately you notice the coolant warning light, visit an auto shop to diagnose the coolant temperature sensor issue.

The professional will first check the coolant’s level before performing a thorough test on the sensor using a voltmeter and might also check for error codes using an OBD scanner. However, if you have a voltmeter, you can perform this test yourself.

The test begins by connecting the voltmeter’s positive lead to the sensor’s signal terminal and its negative lead to the chassis ground. The next step is to cold start the engine and check the readings, which should be between 3V – 4V, depending on the engine’s temperature.

Allow the engine to warm up to its operating temperature, where its voltage reading is expected to drop to 1.2V – 0.5V. If the voltmeter reads 5V, it means there is an open circuit, so check for signal terminal connection and the sensor’s ground contact. If the voltmeter’s reading is 0V, there is a short circuit or no power supply to the sensor.

So, check the connecting wires from the engine control unit (ECU) to the sensor, and check the power supply and ground connection for the ECU. If the voltmeter still reads 0V, then the ECU is the problem and should be fixed.

  • Replace the Coolant Sensor

It is possible that the voltmeter’s readings are more than the range listed above for a cold or hot engine, and the connections are in good condition. If so, you must replace the sensor with a new unit to fix the issue.

Fix a BMW Car Coolant Light

A mechanic should perform the replacement process unless you have the technical experience. The procedure begins by allowing the internal combustion engine to cool for at least fifteen minutes to prevent burn injuries.

The next step is to locate the coolant sensor on the engine block (close to the thermostat housing), then put a draining pan under the car because the coolant may leak out after the sensor is removed.

Slowly detach the wiring connector from the sensor terminal and unscrew the old sensor. Take the new sensor unit and screw it in the clockwise direction, then tighten it using a torque wrench in the same direction.

Afterward, attach the connector to the sensor and start the engine to warm it up. Inspect the new sensor’s performance by observing the dashboard temperature gauge to ensure that it indicates a temperature change.

  • Repair the Cooling System Leakage

The first step to fixing leakage in the cooling system is to detect where the leaks occurred. Finding the coolant leak is not hard because you may notice bubbles in the coolant reservoir or at a hose connection.

However, be careful not to make this inspection when the engine is still hot to avoid experiencing serious burns. If you notice puddles of coolant underneath the vehicle, fluid is dripping from somewhere in the system.

So, trace the fluid to its source and check the hose connections. If you cannot see a leak despite losing coolant, you may have to perform a coolant pressure test. Simply use a pressure tester to gauge the pressure inside the cooling system, and if there is a low pressure, it means there is a leak somewhere.

After detecting the location of the leak, repair the old or damaged component of the cooling system. You may have to replace some of these components with a new one. For instance, a cracked or broken hose, radiator cap, and reservoir tank should be replaced with new ones to end the leakage.

Also, inspect the hose clamps to see if they are tightened or loose, and fix them if needed. Well-tightened clamps help to prevent leaks in the cooling system.

  • Top Up With the Correct Coolant

The use of the wrong antifreeze should be corrected by replacing it with a new coolant. The engine coolant should be replaced after consultation with the car’s manual because the procedure can slightly differ depending on the make and model of the car. Also, the manual helps you to know the exact location of each component. The typical step-by-step process begins with ensuring that the engine is cold, the handbrake has been applied, and the car is in first gear.

Elevate the front of the vehicle carefully on a jack, put the jack stands underneath, and lower it into position. As a precaution, the back wheels should have chocks in place. Remove any underbody shielding, place a container under the radiator, and detach the drain valve. The next step is to flush the system as much as possible to ensure it is empty of coolant.

Locate and remove the reserve tank from its holder, and drain any remaining coolant before putting the tank back in place. Replace the drain valve and refill the system to the base of the filler neck. Lastly, ensure you use the specific coolant brand for your BMW car.


– How Often Should You Inspect Your Engine Coolant?

You should inspect the engine coolant weekly to check its level and color. You should also check the coolant’s level immediately after the coolant pressure light comes on. Turn off the engine and allow it to cool off for some minutes before checking the coolant overflow tank beside the radiator.

– Can Your BMW Car Coolant Level Be Low Without Leakage?

Yes, your BMW car coolant level can be low without leakage, usually due to a damaged head gasket burning up the coolant. The gasket seals the space between the cylinder head and the engine block. If it’s blown, the coolant and engine oil may mix, reducing the coolant’s level.

– Does the Color of the Engine Coolant Matter?

Yes, the color of the engine coolant matters because it indicates the coolant’s health. The coolant should have the same color as it did when you first applied it to the engine. If the color changes to a rusty, reddish-brownish color, this is your cue to replace it.

Cooling System of BMW


Having read this article and the frequently asked questions, you now know why your BMW car coolant’s light is coming on and off.

Here’s a recap of what we have discussed so far:

  • The most common reasons a BMW car coolant light goes on and off are a defective coolant sensor, inaccurate antifreeze, low coolant levels, and leakage in the cooling system.
  • The BMW car coolant trigger light can be fixed by repairing the coolant sensor, replacing the sensor, replacing the coolant, and repairing the leakage in the cooling system.
  • The engine coolant should be inspected weekly to check its level and color, and immediately the coolant pressure light should turn on.

With the information in this article, you can troubleshoot the cause of the coolant trigger light and use the steps provided to fix the problem.

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