This is a fault for the engine coolant temp system. It can mean one of two things- which could be more easily pinpointed if we had freeze frame data that shows at what temperature this fault sets.

First, I would check and see what Engine Coolant Temp (ECT) sensor the car has- it will be in the coolant flange on the drivers' side of the head. It is a 4-pin black plastic connector , and the actual sensor will either be a black plastic (old outdated version) or a green plastic (new updated version). Irregardless of which one you have, remove the electrical connector and make sure that the pins in the sensor are not seeping coolant or looking rusty or oxidised. even the new green sensors have been known to fail.

The ECT sensor is easy to replace, just remove the black plastic "C" clip and pull the sensor upward, make sure to remove the rubber o-ring seal and replace it when you put the new sensor in. The sensor must be seated flat and fully, the "C" clip will sit on top of the brass metal area of the sensor.

The other common cause is the thermostat. If you have the freeze frame data available and the fault sets at a temperature below about 78 degrees, it is likely the thermostat. The engine is watching for the engine to warm up to a certain temperature within a certain time frame, and if it doesn't is assumes a problem (and this is usually caused by the thermostat being stuck slightly open and letting coolant pass from the radiator to the engine, keeping it cooler)

The thermostate is not too hard to replace- you need to follow the coolant hose from the top of the passenger end of the radiator back behind the alternator to the block- the housing is held on with two 10mm head bolts (with a 5mm allen hole as well, take your pick!) Then a large o-ring seal, and the thermostat comes out, also dumping a lot of coolant- have a pan underneath. Seat the new thermostat , then the o-ring seal, and place the housing over it, making sure to keep it flat (finger tighten the bolts evenly first) Then refill the coolant with a 50/50 mix.