There could be several answers to that question. The thermostat could be stuck closed. The reservior could be overfilled. The cooling fan may not be working. The radiator could be stopped up.
In the engine compartment, on the left side of the engine is the coolant reservoir. In the coolant reservoir, there is a sensor at the bottom of the reservoir. This sensor is supposed to let you know when you are low on coolant, but can also become defective, and give you a "false-positive" reading. The only way to know this is to check your coolant when the low coolant light comes on. If there is a significant decrease in coolant, you may have a problem in the cooling system , but if you don't, chances are the sensor isn't working. If the sensor is the problem, you can unplug it from the coolant reservoir, by unscrewing the two screws holding the reservoir to the car's inside firewall, then tilting the back of the coolant reservoir forward until you see wiring coming from the bottom of the reservoir, which is the plug for the low coolant sensor. Unplug the sensor, and the low coolant light won't come on anymore. However, you will now have to make sure you check your coolant level in the reservoir on a regular basis.
It probably has a leak. Here's something to check - the radiator has a tube coming out to the reservoir, so when it gets hot and expands the coolant goes into the reservoir and should go back into the radiator when it cools, but if your reservoir has a crack and the coolant leaks out, well it's gone and can't go back into the radiator - this would cause a lot of loss of coolant.
That is the overflow hose. It comes out at the level of the cap to allow for any extra coolant to escape.
There is a sensor located at the bottom of the coolant reservoir.
disconnect the hose going to the radiator plug the hose coming from the engine side and flush with a garden hose
Overheating due to coolant going to the reservoir and not coming back to the radiator because of the bad cap.
check hoses coming from coolant tank by sqeezing may be worn
This is a indication of a blown head gasket,unless it has been over filled or overheated.If you are getting combustion in the cooling system you will see air coming up through the coolant in the reservoir,be careful it can blow hot coolant out without any warning.Most shops have a tool that will detect combustion gases in the coolant,it should be a inexpensive test.
You have reinstall the head gasket. It has been not tighten enough or damaged. Anyway the replacement was not done properly.
Check coolant level Could be alternator going bad Could be coolant fan going bad
water pump or thermostat housing crack
You will need a intake manifold set and possibly a plenum gasket set.