How Do I Replace Two Thermostats on an Altima?


The cooling system on 2003 to 2005 Nissan Altimas came equipped with two thermostats. Both thermostats provide the same function but in different places: When the engine temperature reaches its operating temperature, the bottom thermostat opens and allows the coolant to flow out of the radiator and into the engine block. The top thermostat opens to allow the coolant to circulate out of the engine block and into the top of the radiator. Once that the coolant flows back into the top of the radiator, the coolant drains back through the radiator and repeats the circulation process.

Things You'll Need

  • Jack
  • Jack stands
  • Drip pan
  • Pliers
  • Flat-head screwdriver
  • 3/8-inch drive ratchet
  • 3/8-inch drive socket set (metric)
  • Flat metal scraper
  • Clean rags
  • Red (high-temperature) silicone
  • New thermostats with gaskets
  • Antifreeze
  • Water
  • Park the Nissan Altima in a safe work area that has a level surface, and shut the engine off. Apply the parking brake and open the hood. Wait a couple of hours for the engine to completely cool down.

  • Check the radiator cap to make sure it has cooled off and that there is no pressure on it. Slowly unscrew the radiator cap and place it in a safe area.

  • Jack the front end of the Nissan Altima up and place the jack stands under the proper front jacking points on both sides of the car. Lower the Altima safely onto the top of the stands and leave the jack in place.

  • Locate the drain valve on the bottom of the radiator. Position the drip pan under the drain valve. Open the drain valve with the pliers and wait for all of the coolant to drain out. Retighten the drain valve. Move the drip pan out of the way.

  • Follow the lower radiator hose from the radiator to where it connects to the lower thermostat housing port. Remove the hose clamp from the hose that secures to the port with a flat-head screwdriver. Pull the clamp back onto the hose. Twist the hose counterclockwise and pull outward on the hose at the same time until the hose comes off the lower thermostat housing port.

  • Remove the two bolts that secure the lower thermostat housing to the engine block with a ratchet and a metric socket. Pull the housing off the engine block and place it on the surface. Pull the old thermostat straight out of the engine block.

  • Slide the new thermostat into the engine block with the thermostat spring inside the engine block. Scrape away any excess gasket from the base of the engine block and from the base of the thermostat housing with a flat metal scraper. Wipe away the excess gasket with a clean rag.

  • Apply a thin bead of red (high temperature) silicone around the base of the thermostat housing. Stick the thermostat housing gasket to the bottom of the housing. Make sure that the holes of the gasket match up with the holes of the housing. The silicone will hold the gasket in place.

  • Lower the thermostat housing back over the thermostat and onto the engine block. Screw the two mounting bolts into the housing and the engine block. Tighten the bolts with the ratchet and socket. Slide the hose back onto the thermostat housing port. Position the hose clamp back over the hose and the port. Tighten the hose down tight with the flat-head screwdriver.

  • Move to the top thermostat and repeat the same exact steps as outlined above for replacing the thermostat. Begin by following the top radiator hose from the radiator to the thermostat housing port. Once both thermostats have been replaced, jack the front of the Altima back up and remove the jack stands. Lower the car to the ground and remove the jack.

  • Fill the radiator with antifreeze and water until the fluid level stabilizes at the top of the radiator fill neck. Leave the radiator cap off and crank the engine. Wait for the fluid level inside the radiator to drop once the engine reaches its operating temperature.

  • Continue adding antifreeze and water to the radiator while the engine is running until the fluid level stabilizes at the top of the radiator again. Look inside the radiator cap and make sure the fluid is moving. This indicates that it is circulating and that both thermostats are open.

  • Continue watching the fluid level inside the radiator for another five or 10 minutes. If the fluid level is still stabilized at the top of the radiator, turn the engine off. If not, continue adding coolant until the fluid level does stabilize at the top of the radiator.

  • Screw the radiator cap back onto the radiator and check the upper and lower thermostat housings for any fluid leaks. Shut the engine off.

Tips & Warnings

  • Be sure to follow the mixing directions on the type of antifreeze that you are using to get the antifreeze-to-water ratio correct.
  • Once the coolant level drops inside the radiator, the top thermostat has opened and is allowing the coolant into the engine block. Once you see the coolant circulating through the radiator, this means the lower thermostat has opened and is allowing the coolant back into the radiator.
  • The red (high temperature) silicone is specially designed for high-temperature components and can be found at most auto parts stores.
  • Always wait for the engine to completely cool to prevent severe burns.
  • Use caution when working under a car that is on jack stands.


  • " Nissan Altima Haynes Repair Manual for 1993 Thru 2006 Covering All Altima Models;" John Haynes; 2007
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