Coolant Temperature Light: What It Means and What to Do
The Coolant Temperature Light means the engine coolant is too hot, and continued driving can warp the cylinder head, blow the head gasket, or seize the engine.
- What it looks like
- A thermometer standing in wavy lines that represent liquid.
- Color
- Red means the engine is overheating. Some vehicles show a blue or green version when the engine is still cold, which is informational and goes out as it warms up.
Also called: engine temperature warning light, overheating light, thermometer in water light, high coolant temperature light, temp light
Updated 2026-11-07 · By CarWhere Research
Why the coolant temperature light comes on
A coolant temperature sensor reports engine temperature to the engine computer and gauge. The cooling system uses a water pump, thermostat, radiator, fans, and pressurized coolant to hold a steady temperature. When temperature passes a safe limit, the light comes on; some engines also reduce power or shut off cylinders to protect themselves.
Steady vs flashing
A blue or green cold-engine light on startup is normal and goes out as the engine warms. A red light that comes on steadily, or flashes on vehicles that use flashing as an escalation, means overheating and requires stopping.
Common causes
- Low coolant level from a leaking hose, radiator, water pump, or heater core
- Thermostat stuck closed
- Failed radiator cooling fan, fan relay, or fan fuse
- Failing water pump or broken drive belt
- Clogged radiator or debris blocking airflow
- Head gasket failure pushing combustion gas into the coolant
- Faulty coolant temperature sensor
What to do when the coolant temperature light comes on
- Turn off the air conditioning and turn the heater to maximum heat and fan to help shed engine heat while you find a place to stop.
- Pull over safely and turn off the engine.
- Wait until the engine has fully cooled, which can take 30 minutes or more, before touching the cap.
- Once cool, check the coolant reservoir level and look for leaks, steam, or a broken belt.
- If coolant is low, add the correct coolant mix to the proper level; use plain water only as a temporary emergency measure.
- If the light returns or there is a major leak, white exhaust smoke, or milky oil, have the vehicle towed rather than driven.
Symbols and colors vary by manufacturer; your owner’s manual has the exact meaning for your vehicle.
Related OBD-II codes
Coolant Temperature Light FAQ
What does the coolant temperature light mean?
The Coolant Temperature Light means the engine coolant is too hot, and continued driving can warp the cylinder head, blow the head gasket, or seize the engine.
Can I keep driving with the coolant temperature light on?
No, when it is red. Pull over as soon as it is safe, shut off the engine, and let it cool. Never open the radiator or coolant reservoir cap while the engine is hot, because pressurized coolant can spray and cause severe burns.
Why is my coolant temperature light blue?
On vehicles that use it, the blue light means the engine is still cold. It is normal after startup and should go out within a few minutes of driving.
Can I add water if my car is overheating?
In an emergency, plain water can get you to safety once the engine has cooled, but it lacks freeze and corrosion protection. Have the system drained and refilled with the correct coolant mix afterward.
Why does my car overheat only at idle or in traffic?
At low speeds the cooling system relies on electric or clutch fans for airflow. A failed fan, relay, or fan clutch often shows up only when the car is stopped or moving slowly.
Other dashboard warning lights
Sources
- 49 CFR 571.101 (FMVSS No. 101) — Controls and displays: telltale symbols and colors
- NHTSA — Check for open safety recalls by VIN
General information written by CarWhere Research, not a diagnosis of your vehicle. When in doubt, stop in a safe place and follow your owner’s manual.