The 2003 Nissan Altima has one thermostat.
In 2003, the Altima was offered with two engines — a 2.5-liter inline-four (QR25DE) and a 3.5-liter V6 (VQ35DE). For both configurations, the cooling system uses a single thermostat mounted in the engine's thermostat housing to regulate coolant flow to the radiator and engine.
Thermostat placement and function
The thermostat sits in the housing where the upper radiator hose meets the engine. It opens and closes to regulate coolant flow, helping the engine reach and maintain its optimal operating temperature. Most Altimas also include a bypass pathway to ensure coolant circulation during engine warm-up.
Location on the 2.5L QR25DE engine
On the 2.5L engine, look for the thermostat housing at the end of the cylinder head near the upper radiator hose. The housing may integrate a temperature sensor and a removable cover.
Before inspecting or replacing the thermostat, ensure the engine is cool and the battery is disconnected to avoid injury.
- Safely access the engine bay and identify the thermostat housing connected to the upper radiator hose.
- Disconnect the negative battery cable.
- Drain a portion of coolant from the radiator to prevent spills.
- Loosen and remove the hose clamps, then detach the upper radiator hose.
- Remove the thermostat housing bolts, remove the housing, and extract the thermostat.
- Clean mating surfaces, inspect the thermostat for sticking, and replace with a new unit and gasket.
- Reinstall the housing and hose, refill coolant, and bleed air from the system.
- Start the engine and check for leaks and proper thermostat operation.
Before replacing or inspecting, ensure the engine is cool and the battery is disconnected. Steps:
Note: If you are unsure or the vehicle overheats, seek a professional mechanic for service.
Summary
The Altima’s cooling system in 2003 uses a single thermostat, regardless of engine choice. Replacing or inspecting it follows standard maintenance steps for the QR25DE 2.5L engine or the VQ35DE 3.5L engine.