The thermostat on the engine block of a 2000 Honda Civic regulates the flow of coolant into the engine depending on the temperature of the coolant. If the thermostat fails, then it won't open and close but will instead stay stuck open or closed. This will keep the heat inside of the coolant, which is the opposite thing that you want, and could overheat the engine. Because of this, you want to remove and replace the thermostat as soon as possible to avoid damaging the engine.
Wait for the Civic's engine to cool for at least four hours before working on the thermostat. Put a drain pan on the ground beneath the radiator, then open the petcock located on the bottom of the radiator using your hands. Shut the petcock once the radiator has emptied completely.
Remove the hose clamp leading into the thermostat, which is located on the top of the engine, using the pliers. Pull the hose off of the thermostat. Unbolt the thermostat housing from the engine with the 3/8-inch ratchet, extension and sockets, then pull it up and off of the thermostat. Pull the thermostat out of the engine and discard it. Discard the thermostat housing seal on the thermostat housing.
Install the replacement thermostat in the engine, with the bump on the inside of the thermostat sticking up toward the hood. Install the replacement thermostat gasket in the thermostat housing. Install some anti-seize compound onto the threads of the thermostat housing bolts. Bolt the thermostat housing cover onto the engine with the 3/8-inch ratchet, extension and sockets.
Install the hose onto the thermostat housing, using the pliers to reposition the clamp. Open the radiator cap. Fill the radiator with the 50-50 pre-diluted coolant.
Start the Civic with the key, and then put the heater to its hottest position. Let the car idle for at least 15 minutes, then turn it off and let it sit for an hour. Check the radiator fill level and then top it off with the pre-diluted coolant if necessary.
