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Is coolant the same as diesel exhaust fluid?


No, coolant and diesel exhaust fluid are not the same. Coolant and diesel exhaust fluid have different compositions and purposes in engines.

Coolant, also known as antifreeze, is a liquid that runs through the engine and radiator to keep the engine from overheating. It is typically composed of a 50/50 mix of ethylene glycol and water, plus additives like corrosion inhibitors. Coolant flows through the engine block and radiator to absorb excess heat and prevent overheating.

Diesel exhaust fluid (DEF) is a liquid that is sprayed into the exhaust stream of diesel engines to lower emissions. It is composed of urea and deionized water. DEF helps convert nitrogen oxides (NOx) in diesel exhaust into nitrogen gas and water vapor, making the exhaust less harmful. DEF is not used for cooling purposes.

What is Coolant?

Coolant, sometimes referred to as antifreeze, is a heat transfer fluid used to regulate engine temperature in internal combustion engines. Its main functions are:

– Prevent engine overheating by absorbing excess heat from the engine and transferring it to the radiator
– Prevent engine freezing in cold temperatures by lowering the freezing point of the coolant mixture
– Prevent corrosion inside the engine cooling system

The most common type of coolant is a 50/50 mixture of ethylene glycol and water. Ethylene glycol has a lower freezing point than water, allowing the coolant mixture to stay liquid at lower temperatures. Other additives like corrosion inhibitors are also added to protect the engine.

Here are some key facts about engine coolant:

Composition Typically a 50/50 mix of ethylene glycol and water plus additives like corrosion inhibitors
Color Usually green, red, orange, yellow, or blue depending on brand
Temperature range -34°F to 265°F (-37°C to 129°C)
Purpose Absorb excess heat, prevent overheating, lower freezing point, prevent corrosion
System Circulates through engine block, radiator, heater core, hoses, water pump

Coolant flows from the radiator into the engine block, absorbing heat from the engine. It then cycles back to the radiator to dissipate the heat outside the vehicle. This constant flow and heat transfer regulates engine temperature.

What is Diesel Exhaust Fluid?

Diesel exhaust fluid (DEF) is a liquid solution used to lower emissions in diesel engine exhaust systems. It is composed of urea and deionized water.

DEF serves an important emissions control purpose in diesel vehicles. When injected into the exhaust stream, it breaks down harmful nitrogen oxide (NOx) emissions into nitrogen gas and water vapor.

Here are some key facts about diesel exhaust fluid:

Composition 32.5% high purity urea, 67.5% deionized water
Color Clear liquid, sometimes dyed blue
Fluid type Non-toxic, non-polluting, non-flammable
Tank capacity 5-10 gallons typically
Purpose Lower NOx emissions by converting to nitrogen gas and water
System Injected into exhaust stream before catalytic converter

DEF helps diesel vehicles meet stringent emissions standards for nitrogen oxides and other pollutants. The urea in DEF breaks down NOx through a chemical reaction facilitated by the catalytic converter. Using DEF can reduce NOx emissions by 90% or more.

Key Differences Between Coolant and DEF

While both coolant and DEF are liquids used in vehicle engines, they serve very different purposes:

Composition: Coolant is ethylene glycol and water. DEF is urea and deionized water.

Purpose: Coolant absorbs engine heat to prevent overheating. DEF reduces NOx emissions in diesel exhaust.

Color: Coolant is often brightly colored. DEF is clear or dyed blue.

System: Coolant circulates through the engine and radiator. DEF is injected into the exhaust.

Temperature: Coolant operates from -34°F to 265°F. DEF freezes around 12°F.

Consumption: Coolant is reused for years. DEF is continually consumed and refilled.

Flammability: Coolant is flammable due to ethylene glycol. DEF is non-flammable.

So in summary, while both are liquid solutions for engines, coolant and DEF serve completely different functions and should never be confused or substituted for one another. Mixing the two fluids could seriously damage the engine.

Can Coolant and DEF Be Mixed?

No, coolant and diesel exhaust fluid should never be mixed together or used interchangeably in vehicle engines.

Coolant and DEF are completely incompatible fluids with different compositions:

Coolant DEF
Ethylene glycol Urea
Corrosion inhibitors Deionized water

Mixing the two fluids could have the following negative effects:

– Crystallization as the urea in DEF solidifies due to ethylene glycol
– Corrosion and damage to coolant system components not designed for urea
– Reduced performance of DEF system due to dilution
– Engine overheating if used as coolant
– DEF system clogging if used as DEF

For these reasons, coolant and DEF have separate tanks and warning systems in place. Mixing DEF and coolant will almost certainly damage expensive engine components. Always use the correct fluid for the system. Never try to substitute or mix them.

Checking Coolant and DEF Levels

Coolant and DEF levels should be checked regularly in diesel and gasoline engines:

Coolant Level:

– Check coolant level cold before starting the engine
– Level should be at the “Full” or “Max” line on reservoir
– Top up with the correct diluted coolant mix if low

DEF Level:

– Check DEF level gauge on dash
– Refill tank if level is low
– Only use branded DEF products like BlueDEF that meet ISO 22241
– Never use water or any other fluid as substitute

Neglecting fluid levels can lead to major engine damage from overheating, emissions failures, or component crystallization. Both coolant and DEF reservoirs should have a cap or screen to prevent contamination. Only use the designated fluid in each system.


Coolant and diesel exhaust fluid serve very different purposes in engine systems. Coolant is an ethylene glycol/water mix that controls engine temperature. DEF is a urea solution that reduces diesel emissions. While both are liquids, they have incompatible chemistry and should never be mixed or substituted. Using the wrong fluid can badly damage the engine. Always check coolant and DEF levels regularly and use the designated fluid for each system. Proper use of coolant and DEF is essential to keep diesel and gasoline engines running cleanly and efficiently.