Short answer
The red temperature light means the engine is overheating or its coolant is critically low. Pull over as soon as it is safe and switch the engine off. Do not open the radiator or reservoir cap while it is hot. Driving on risks a warped cylinder head or a blown head gasket, a far bigger repair than the cause.
- Also shown as:
- Coolant temperature light · Engine overheating light · Coolant light
- What it looks like:
- A thermometer standing in wavy lines of liquid. Red means hot; on some cars a blue or green version means the engine is still cold.
Is it safe to keep driving?
No. Pull over, switch off, and let it cool. Continuing to drive an overheating engine risks a warped cylinder head or a blown head gasket.
Stop driving now if:
- The red light is on, or the temperature gauge is in or near the red.
- Steam rises from under the hood, or you smell something sweet and hot.
- The heater suddenly blows cold air while the engine is hot — a sign the coolant is very low.
- The engine starts knocking, pinging or losing power.
What causes the engine temperature / coolant warning light?
Most common first. Symbols and meanings vary by manufacturer; your owner's manual has the definitive meaning for your model.
- Low coolant from a leak. Hoses, the radiator, the water pump seal and the heater core all leak with age. Look for a sweet smell and a green, orange or pink puddle under the front of the car.
- Cooling fan not running. If the car overheats in traffic but cools down on the open road, airflow is the problem: a failed fan motor, relay or fuse. At speed, air rushing through the radiator hides it.
- Thermostat stuck closed. The thermostat holds coolant in the engine until it warms up. Stuck closed, it keeps hot coolant from reaching the radiator, so the temperature climbs quickly after a cold start.
- Failing water pump. The pump circulates the coolant. Signs are a drip or crust near the pump, a whine or grind from the front of the engine, and overheating at any speed.
- Clogged radiator, collapsed hose or trapped air. A radiator clogged inside or out, a soft hose that collapses under suction, or an air pocket left after coolant work can all stop the flow even with the level full.
- Head gasket failure. Bubbles in the coolant reservoir, white sweet-smelling exhaust smoke, or coolant that disappears with no visible leak point here. It is often the result of an earlier overheat, not the first cause.
What should you do right now?
- Turn the heater on full and the A/C off — it pulls heat out of the engine — then pull over as soon as it is safe.
- Switch the engine off. You can open the hood to let heat out, keeping clear of the fan, which on many cars can start by itself.
- Never open the radiator cap or the coolant reservoir cap while the engine is hot: the coolant is under pressure and can scald.
- Wait until the upper radiator hose is cool to the touch, which can take well over half an hour.
- Check the coolant reservoir against its MIN and MAX marks. If it is low, top up with the coolant type named in the manual; plain water only to get you to a shop.
- If the level was very low, do not just top up and carry on — find the leak before it empties again.
What does the fix usually involve?
A shop pressure-tests the cooling system to find leaks, checks that the fans switch on and that the thermostat opens, and may test the coolant for exhaust gases to rule a head gasket in or out.
Common repairs: a hose or clamp, a thermostat, a fan relay or motor, a radiator or a water pump. On some engines the water pump is driven by the timing belt and is usually replaced together with it.
If the engine overheated badly, ask the shop to check the cylinder head for warping and the head gasket for leaks before you rely on the car again.
After any cooling repair, the system has to be bled of trapped air. Air left behind is a common reason a car overheats again right after a repair.
How much does it cost to fix?
$20 – $2,000
A hose or thermostat at the low end; a water pump, radiator or head gasket at the high end.
A typical range, not a quote. Actual cost depends on the car, the shop and the fault.
Common questions
Can I add water if the coolant is low?
In an emergency, yes, once the engine is cool. Then have the system topped up or refilled with the right coolant mix: water alone boils at a lower temperature, freezes in winter and does not protect the engine against corrosion.
Why does my car overheat in traffic but not on the highway?
At highway speed, air is forced through the radiator. In traffic the electric cooling fan has to do that work, so overheating only when stopped or crawling usually points at the fan, its relay or its fuse.
What does a blue temperature light mean?
On many cars a blue or green thermometer at start-up means the engine is still cold. It goes out as the engine warms up. It is not a fault; just avoid hard revving until it goes out.
Is it safe to open the hood when the engine is overheating?
Opening the hood is fine and lets heat escape; keep hands and loose clothing away from the fan and belts. What is not safe is opening the radiator or reservoir cap before the engine has cooled.
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