Parts for your 2005 Subaru Impreza-Thermostat

2005 Subaru Impreza Thermostat — What it does and when to replace it

Yes, a thermostat is absolutely fitted to the 2005 Subaru Impreza. Technical references including the Subaru Factory Service Manual (STIS) for the 2005 model year and popular workshop guides like the Haynes manual confirm the EJ-series engines (2.0 and 2.5, including WRX/STI variants) use a wax‑pellet thermostat mounted at the water pump outlet, in the lower radiator hose housing. Subaru parts catalogues list the genuine thermostat (commonly 21200AA072) with its sealing ring (21236AA010), further cementing that it’s a standard, serviceable component.

In everyday driving, the thermostat’s job is to get the boxer engine up to temperature quickly, then keep it there. It stays closed while the coolant is cold so the engine warms up fast, reducing wear and fuel use. Once it reaches operating temp, it opens progressively to send coolant through the radiator, keeping temps steady whether cruising to the shops or climbing an alpine pass.

For a 2005 Impreza, the thermostat isn’t a routine “change by date” item, but it’s smart to replace it when doing major cooling work—like a water pump or timing belt service—or any time there’s been an overheating episode. Always go for a genuine‑spec thermostat and new O‑ring to avoid fitment dramas and erratic temperature behaviour.

• Common signs it’s on the way out:
  • Slow warm‑up or the gauge sitting low on the open road
  • Overheating at speed or temps that surge up and down
  • Heater going cold at idle, then hot when revved
  • Lower radiator hose staying cold when the engine is hot

Replacement on the EJ engine is straightforward for a competent home mechanic: drain the coolant, pop the lower hose off the water pump housing, remove the cover, swap the thermostat and O‑ring, and refit the cover, tightening to the factory spec (don’t overdo the small bolts). Refill with Subaru‑approved coolant mix and bleed air with the heater set to hot. Let the fans cycle a couple of times, top up the radiator and overflow bottle, and check for leaks. If the old coolant looked rusty or sludgy, a proper flush is worth the effort. After a day or two of driving, recheck levels under the bonnet.

Kept healthy, the thermostat helps the 2005 Impreza run sweet as—stable temps, comfy cabin heat, and better fuel economy.

Popular questions

Where is the thermostat on a 2005 Subaru Impreza?
It sits in the lower radiator hose housing on the front of the engine, integrated with the water pump outlet. Access is from underneath or front‑on once the lower hose is off, so have a drain pan handy.

Should the thermostat be replaced with the timing belt or water pump?
It’s good practice. While Subaru doesn’t mandate it by time or kilometres, replacing the thermostat (with a new O‑ring) during a timing belt/water pump job saves on labour later and helps avoid temperature hiccups from a sticking old unit.

What coolant should be used after thermostat replacement?
Use Subaru‑approved coolant mixed to the correct ratio with demineralised water. Don’t mix coolant types, if changing type, flush thoroughly first. After refilling, bleed the system carefully to avoid air pockets.