Yes, you need to replace the coolant when changing the thermostat. This is because the coolant must be drained from the engine during the thermostat replacement process, which may result in some loss of coolant. Additionally, the coolant may come into contact with impurities when removing the thermostat. Therefore, it is generally recommended to replace the coolant along with the thermostat. Method for Replacing Coolant: Before completely replacing the coolant, conduct a thorough inspection to check for any signs of leaks or cracks in the pipes, with a particular focus on the five-way hose. The five-way hose is named for its five connections to different parts of the engine. Coolant flows through the five-way hose and is distributed to various parts of the car to perform its function. If there are any signs of coolant leakage in this area, replace the hose or reseal the connections as needed. Drain the old coolant, then flush the fluid channels with clean water. Add clean water to the coolant reservoir and continue filling until water flows continuously through the engine cooling system. Start the engine and let it idle for 3 to 5 minutes to circulate the water. Initially, the water draining from the reservoir may appear slightly pink. Continue adding clean water until the water runs clear. Remember to remove the heater hose and drain all water from the heater core. After draining for about an hour, add the new coolant through the radiator hose to allow it to flow quickly into the radiator. Then, add another bottle of coolant to the reservoir until it is nearly full. Start the engine and let it run for about 10 minutes. As air is purged from the cooling system, the coolant level will drop. Top up the coolant until it reaches the "MAX" mark on the reservoir.
