Q: How to replace a thermostat in a Nissan Altima?
A: Drain the cooling system and save the coolant if it's in good condition. Remove the fresh air intake duct, lower radiator hose, and air filter housing. Disconnect hoses from the coolant outlet and heater pipe, installing a new O-ring on the heater pipe end. Remove the housing cover, tap it loose if stuck, and clean mating surfaces. Remove the thermostat or water control valve, install a new sealing ring, and insert the valve into the housing with the air bleed facing up for the thermostat. Apply RTV sealant for the thermostat cover or use a new gasket for the water control valve. Tighten bolts evenly, then refill the cooling system. For V6 engines, remove the water pump drain plug and Coolant Reservoir, disconnect the IVT valve connector, and the lower radiator hose. Remove the thermostat housing/thermostat assembly and replace it if the thermostat is defective. Install everything in reverse, position the jiggle valve correctly, refill the cooling system, start the engine, and check for leaks and proper thermostat operation.