When you think that there is an issue with the cooling system then inspect the level of the coolant, drivebelt tension and the working of the temperature gauge. If the engine appears to be slow in warming up, by reference to the heater output or temperature gauge operation, it can be concluded that the thermostat is stuck open, and a new one is required. If the engine emits heat, check the upper radiator hose; if it is not hot and the engine is, the thermostat is most likely stuck shut and is not allowing coolant to flow to the radiator in order that it might cool and return to the engine and needs replacement. Do not operate the vehicle without a thermostat because doing that will force the computer into an open-loop mode, which results in high emissions and poor fuel economy. It is possible to check the flow of coolant by touching the upper radiator hose; if it is hot, then the thermostat is open. This is especially so because the drivebelts are also likely to be damaged when exposed to coolants thus cover them with kitchen plastic wrap. Remove the cable from the negative terminal and flush out the coolant if any then drain the cooling system, the coolant used should be reused if it is good. In particular engine models, the necessary air ducts should be removed and tracing the radiator hose to the thermostat housing should be done. Undo the hose clip and pull off the hose and if worn or damaged then fit a new one. If the housing cover of the thermostat has some signs of corrosion, then it is advisable to change it. For V6 models: take the screws off to remove the thermostat cover plate and pull it off, taking particular note of the thermostat's position. Remove all old gasket deposits from the housing and cover and fix the new thermostat properly in its place then fit the housing cover with a new gasket. For V8 models, one has to proceed in a similar fashion, but has to be more careful and remember to undo any brackets, which might be present. Put the radiator hose back into its place, top up the cooling system again and join the battery terminals. Starting the engine and allowing it to warm up, help the thermostat to open so that it's operations can be checked for any signs of leakage.