What Does the Coolant Temperature Light Mean?

The coolant temperature warning light (a thermometer in liquid) means your engine coolant temperature has exceeded the safe operating range, typically above 110 degrees Celsius. Your engine is overheating.

This is a serious warning. Continuing to drive an overheating engine can warp the cylinder head, blow the head gasket, or seize the engine — all extremely expensive repairs.

What to Do When Your Engine Overheats

Take these steps immediately:

1. Turn off the air conditioning — it adds load to the engine.
2. Turn the heater to maximum — this acts as a secondary radiator, drawing heat from the engine.
3. Pull over as soon as it's safe.
4. Do NOT open the radiator cap when hot — pressurised coolant will spray out and cause severe burns.
5. Let the engine cool for at least 30 minutes.
6. Check the coolant level in the overflow/expansion tank (not the radiator cap).
7. If coolant is low, add water or coolant. If it's empty, do not restart without adding fluid.
8. If the temperature drops back to normal, you may drive carefully to a mechanic. If it rises again, stop and call for a tow.

Common Causes of Engine Overheating

Why engines overheat:

• Low coolant level — Leak in the system or evaporation over time.
• Failed thermostat stuck closed — Prevents coolant from flowing to the radiator.
• Failed water pump — The pump that circulates coolant has worn out or the impeller has corroded.
• Blocked radiator — External debris (bugs, dirt) or internal scale/sludge blocks airflow or coolant flow.
• Failed radiator fan — Electric fans that don't activate when needed. Common in stop-and-go traffic.
• Blown head gasket — Combustion gases pressurize the cooling system, pushing coolant out.
• Coolant leak — Cracked hose, failed water pump seal, or corroded radiator.

Related OBD-II Codes

Common overheating-related codes:

• P0115 — Engine Coolant Temperature Circuit
• P0116 — Engine Coolant Temperature Range/Performance
• P0117 — Engine Coolant Temperature Low
• P0118 — Engine Coolant Temperature High
• P0128 — Coolant Thermostat Below Regulating Temperature
• P0217 — Engine Overtemperature Condition
• P0480 — Cooling Fan Relay Circuit

Repair Costs

Australian estimates:

• Coolant top-up: $15–$30
• Thermostat replacement: $150–$350
• Radiator hose replacement: $80–$200
• Water pump replacement: $300–$800
• Radiator replacement: $400–$900
• Radiator fan motor: $200–$500
• Head gasket replacement: $1500–$3500