What Does This Error Mean?

Subaru coolant temperature warning means the engine is overheating — stop and switch off the engine immediately. Overheating a Subaru EJ-series Boxer engine is particularly serious because head gasket failure is already a known weakness — overheating significantly accelerates this damage.

Affected Models

Subaru Outback
Subaru Forester
Subaru Impreza
Subaru Legacy
Subaru XV / Crosstrek
Subaru WRX

Common Causes

Low coolant level from a slow leak — common at hose connections and the water pump on higher-mileage Subarus
Head gasket failure causing coolant loss — the EJ25 SOHC engine is particularly prone to external coolant leaks from the head gaskets
Thermostat stuck closed — prevents coolant circulation to the radiator
Radiator fan failure — electric fan relay or motor fault
Water pump failure or worn impeller

How to Fix It

Stop safely and switch off the engine.

Pull over as soon as it is safe and turn off the engine immediately. Using the heater on maximum can help briefly draw heat away from the engine as you find a safe stopping point. Do not continue driving — EJ engine head gasket damage begins within minutes of severe overheating.
Wait 30 minutes before touching the cooling system.

Subaru cooling systems are pressurized — opening the coolant cap on a hot engine releases scalding steam and coolant. Wait at least 30 minutes with the hood open for ventilation before approaching the radiator cap or overflow tank. Place a thick cloth over the cap and release pressure slowly before fully removing.
Check the coolant level.

After the engine has cooled, check the coolant overflow tank (translucent plastic, usually on the right side of the engine bay) against the MIN and MAX marks. If low, refill with the correct Subaru coolant — Subaru uses a blue Super Coolant (phosphate-free, OAT type) in most markets; mixing colors or types can damage the cooling system. If the level is repeatedly dropping, there is a leak that must be found.
Inspect for a head gasket leak.

Subaru EJ25 SOHC engines (non-turbocharged Outback, Forester, Legacy, Impreza 2000–2012) are notorious for external head gasket leaks that slowly weep coolant. Look for dried orange or brown staining on the outside of the engine block where the cylinder head meets the block — this is coolant residue from a seeping gasket. A combustion gas test of the coolant confirms internal head gasket failure.
Check the thermostat and radiator fan.

A thermostat stuck in the closed position causes the engine to overheat quickly from startup even with a full coolant system. If the lower radiator hose stays cold while the engine overheats, the thermostat is not opening. With the engine at operating temperature, the electric radiator fan should be spinning — if not, check the fan relay and fuse in the engine bay fuse box.

When to Call a Professional

Any overheating event on a Subaru with an EJ-series engine (Outback, Forester, Legacy, Impreza 2003–2012) warrants a head gasket check. The cost of checking is far less than the cost of a full head gasket repair if damage is discovered later.