Coolant, also called antifreeze, circulates through the engine and radiator to keep temperatures in a safe range. Without enough of it, an engine can overheat quickly, and overheating is one of the more expensive problems a car can have. This article explains what coolant does, the signs of low coolant and how to handle it. For anything specific to your car, rely on the owner’s manual.
What Does Coolant Do?
- Removes heat: It carries heat from the engine to the radiator, where it is released.
- Prevents freezing: The antifreeze component lowers the freezing point of the liquid in cold weather.
- Raises the boiling point: A proper mix boils at a higher temperature than plain water.
- Protects against corrosion: Additives help protect metal parts and seals in the cooling system.
So yes, coolant is necessary. Plain water lacks these protections.
Signs of Low Coolant
- Temperature gauge reading higher than normal, or an overheating warning light
- Coolant level below the minimum mark on the reservoir
- Steam or a sweet smell from under the hood
- Puddles under the car, often green, orange, pink or blue
- Cabin heater blowing cold or lukewarm air
- Engine noise or reduced performance as temperatures rise
What Happens When Coolant Is Low
With too little coolant, heat is not removed effectively. The engine may overheat, which can damage parts such as the head gasket, cylinder head and in severe cases the engine itself. If your gauge climbs, pull over safely, switch the engine off and let it cool. See engine oil overheating for how overheating affects the oil, too.
Why Does a Car Lose Coolant?
Coolant is in a sealed system, so a falling level suggests something is wrong. Possible causes include:
- Leaking hoses, radiator, water pump or heater core
- A loose or faulty radiator cap
- A failing head gasket letting coolant into the cylinders or oil
- Evaporation or small losses over a long time
Topping up repeatedly without finding the cause only delays the problem.
Are All Coolants the Same?
No. Coolants differ in chemistry and color, and color alone is not a reliable guide. Using the type your manufacturer specifies matters, because some mixes can react badly or protect less well. Many coolants are sold as concentrate to be diluted or as a pre-mixed 50/50 solution; check the label and manual.
Checking and Topping Up Safely
- Park on level ground with the engine completely cool
- Find the translucent overflow reservoir and compare the level to the MIN and MAX marks
- Never open the radiator or pressurized cap on a hot engine, since scalding liquid can spray out
- If low, add the specified coolant type and mix to the reservoir mark
- Recheck over the next few days and look for leaks
If the level drops again, have the system pressure-tested by a mechanic.
Can You Use Water in an Emergency?
Water can be a short-term emergency measure to get to a safe place, but it should be drained and replaced with the proper mixture as soon as possible, particularly in cold weather where it may freeze.
Replacement Intervals
Coolant degrades over time. Intervals vary widely by vehicle and coolant type, so follow your owner’s manual. Coolant also appears on the typical car maintenance schedule. Also see our engine troubleshooting guide for other checks worth making regularly.