Check the coolant level (chapter "Cooling system") and add coolant or water if necessary.
If the engine is hot and the coolant level is normal, you should check whether the electric cooling fan works when the ignition is on. If it does, despite the hot engine, you should refer to the list of faults for the cooling fan (chapter "Cooling system").
Last possibility: one of the two sensors of this indicator is faulty (see next section).
Troubleshooting the Coolant Indicator
Indicator Check: When the ignition is turned on, the coolant indicator blinks to check the operation. If this does not happen:
Check the corresponding fuse (chapter "Body electrical system").
If it is in good condition, you should remove the instrument cluster, remove and check the indicator.
If the bulb is OK, you need to check the wire or replace the mini-system unit (see section "Disassembly" in this chapter).
Checking the temperature sensor in the engine block: if the indicator blinks even when cold or at normal operating temperature (working) engine, then the temperature sensor may be faulty.
Disconnect the multi-pin plug at the temperature sensor: the light should go out.
Wire to coolant indicator (the illustration on page 236 shows exactly what this indicator is) connect to ground: the light should blink again.
If this happens in both cases, then the bulb is working properly; then the cause should be sought in the sensor or wiring. Check or replace. Or in the coolant level sensor.
Checking the coolant level sensor in the expansion tank: the sensor is installed at the bottom of the expansion tank and is rigidly connected to it.
If the indicator blinks despite the correct fluid level, you should disconnect the wire from the sensor.
If the flashing stops after this, you need to clean the sensor inside the tank or replace the tank.