Temperature Warning Light On

Coolant Sensor
Light On for Engine Coolant Temperature Hot

All warning lights are meant to alert you that a problem in one of the operation systems in your car has or is malfunctioning. The engine temperature warning light is conveniently placed in the instrument cluster display panel. Warning lights generally come in two colors, orange and red. Red is reserved for the more serious malfunctions that need attention immediately while orange is meant to be serviced at your earliest convenience. When a particular system fails a warning light device has been designed into most car systems such as brakes, engine, transmission, seat belts, air bag, vital fluid levels and more. All engine temperature warning lights are colored red because if this light illuminates the engine is overheating. If left unattended the engine will suffer irreversible damage. When the engine overheats the engine pistons expand causing the cylinder wall to become scored. When cylinder wall scoring occurs the piston rings cannot seal to the wall allowing combustion bypass when the engine cools.

This is why after an engine has been overheated it can have low power and cause the engine to burn oil. This condition is called blow-by and cannot be repaired unless the engine is rebuilt or replaced. If you think this condition has occurred to a car an engine compression test is needed. If this test shows that the compression is below the normal range you can suspect engine overheating has occurred. In sever engine overheating conditions the internal engine parts can swell forcing the engine to stop turning or seize. If the engine seizes due to overheating the internal parts of the engine are damaged and replacement or disassemble for repair is needed. A common cause for engine overheating is lack of coolant in the engine cooling system. When lack of coolant is present the heat transferring properties is lost. This absence of heat transfer is what causes the engine overheating. A car radiator cannot work if there is no coolant inside the system. The coolant level should be the first check after an engine has overheated. Never check engine coolant until the engine has cooled completely, preferably after the engine has cooled over night.

Another popular reason for engine overheating is the thermostat can stick closed not allowing the coolant to flow to the radiator. The engine thermostat is designed to open allowing antifreeze/coolant to flow to the radiator when the thermostat inside the engine reaches about 198F - 99C. Even if the coolant is at the proper level this condition can occur. Only check for this condition after the engine had cooled down a safe temperature. Then check the temperature of both the upper and lower hoses. If the top hose is warm and the bottom hose is cool you can suspect the thermostat has stuck and needs to be tested or replaced. If your car is equipped with radiator cooling fans it is vital these fans work properly. If the cooling fan fail to operate the heat from the radiator will not be removed from the engine coolant causing the engine to overheat. If the engine is overheating the cooling fans should be on in full force. If the cooling fans have not come on the electrical system that power the fans must be checked.

If an engine overheating condition has occurred one of the most popular causes for this condition is cooling fan motor failure. These fan or fans use a high amperage draw and are prone to failure. Other popular reasons for failure is a fuse or relay operation failure. Use a test light or voltmeter to test these circuits and replace burnt or shorted components with new. In most cases engine overheating is obvious and can be detected by additional engine noise, poor running or low power even before the engine temperature warning light is illuminated. If you car is equipped with both a temperature warning light and a engine temperature gauge it is a good idea to periodically inspect the gauge while driving.

When cars are designed with a temperature gauge along with a warning light it is a "redundant" system and is used as a failsafe. In this case the warning light works in correlation with the gauge and will only illuminate when the gauge is in the upper range of the gauge span.  Never depend on a warning light to illuminate when the temperature gauge is in the "hot" range. If the temperature gauge is in the "hot" range shut the engine off in a safe place on the highway. Allow the engine to cool before inspection. Sometimes the temperature gauge can go to the hot range and then back down to the cool range if this condition happens check the electrical contact at the temperature gauge sender electrical connection. If this contact is loose it will cause intermittent high resistance allowing the temperature gauge circuit to connect and then go to high resistance when the engine it running due to engine vibration. When the connections goes to high resistance the temperature gauge will go to the "hot" position ever thought the engine is not hot. Always make sure the connection is tight and making good contact with no corrosion. Even if the connection is tight corrosion can cause high resistance when the engine is running.

If the cooling system radiator is plugged it can cause the engine to overheat. A radiator can become plugged by corrosion or debris in the cooling system. If either of these conditions occur the radiator cannot pass coolant through the cooling tube which are connected to the cooling fin of the radiator. In turn the engine coolant will be hot as it re-enters the engine block and cylinder heads causing the engine to overheat. To check for this condition (after the engine has cooled overnight) remove the radiator cap or upper radiator hose. Then with a flashlight shine the beam into the radiator and down or over to the cooling tube openings. If the opening look clogged the radiator must be replaced or serviced (boiled out) by a radiator repair shop. Some of the overly un-popular causes for engine overheating are internal engine gasket failures such as a head or intake manifold gasket failure. This condition is accompanied by low coolant level in most cases.

In extreme cases the engine block or cylinder head can develop a crack due to manufacturing flaws. This condition will present itself in much the same way as a failed internal engine gasket but the remedy is much most expensive. Usually when this condition occurs the engine block or cylinder head needs replacement. Anytime your car engine fails you must consider he engine mileage as it might be more beneficial to replace the engine as a unit opposed to repairing an high mileage engine. Also if the car has seen better days it might be time to purchase a different car. This decision can be difficult especially if the car is still in good condition, just old.

Do not attempt to open the engine cooling system at anytime unless the engine has cooled overnight. The engine cooling system is contained under pressure, usually about 18 psi. This coolant is hot about 230F or 102C and it will burn you or anyone standing with range of coolant projecting from the engine compartment. Sometimes a car hood can be hot from a result of an overheating engine. Before opening the hood touch the hood quickly to measure the hood temperature before opening. Never open a overheated hood, allow a cooling period, preferably over night.

Common causes for the engine temperature light to be illuminated

  • Low Coolant Level - When the temperature light comes on the first thing that should be checked is the coolant level. If this level becomes low due to a leak the engine can over heat

  • Failed Coolant Fan Operation - The cooling fan must be operational or the engine will overheat causing the engine temperature light to come on.

  • Plugged Radiator - The car radiator must be operational, free form corrosion with clean cooling fins

  • Failed or Stuck Thermostat - If the engine thermostat fails to open it will not allow the coolant/antifreeze to flow from the engine to the radiator

When parking the car after the engine temperature light has come on check under the car to see if engine coolant/antifreeze is leaking onto the ground. This will help you discover the area and the cause the car engine is overheating. Let it be known if the temperature light has come "on" coolant (engine overheated) can be force out of the coolant reservoir no matter what the reason of the engine overheating. Engine coolant or antifreeze will be force out of the coolant reservoir when the engine runs hot or over heats because the coolant expands beyond the containment of the reservoir. After the engine cools inspect the location of the coolant leak. If the only coolant leak you can find is from the coolant reservoir due to an overflow from reservoir then chances are the cause of the overheating could be a failed thermostat, plugged radiator or the cooling fans not coming on.

If you notice coolant leaking from places other than the coolant reservoir further inspection is needed. Check components like the water pump, radiator, freeze plugs (located on the lower portion of the block) and coolant/heater hoses. Also engine gaskets can fail such as the intake manifold or head gaskets. Either of these gaskets has the ability to leak internally. If either a intake manifold or head gasket fails it has the ability to leak to the internal part of the engine.  If this condition is allowed to go unattended to the engine could suffer internal damage. This damage is due to the lack of lubrication properties when engine coolant/antifreeze is introduced to the motor oil.

Occasionally the temperature sensor itself can become defective illuminating the high engine temperature light even though the engine is not overheating. To test the temperature sensor a volt/ohm meter is needed. Once the engine is cool remove the sensor. Note: drain the cooling system first because engine coolant will leak from the cylinder head or intake manifold were the sensor is located. Next test the resistance of the sensor by attaching one of the test leads to the body or terminal of the sensor. Then attach the remaining lead to the remaining terminal of the sensor. This reading should be within factory specifications. If the sensor is not within range the sensor has failed and needs to be replaced. Most temperature sensors are designed with one or two electrical terminals. If the temperature sensor you are testing has just one electrical terminal use the side of the sensor as the remaining terminal. When reinstalling the temperature sensor always inspect the sealing washer or pipe thread sealant/tape to ensure proper seal to the cylinder head or intake manifold.

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Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-16)