Parts for your 2012 Subaru Impreza-Thermostat housing

2012 Subaru Impreza thermostat housing: what it does and how to look after it

Thermostat housing is absolutely relevant on the 2012 Subaru Impreza. On FB-series 2.0L engines fitted to this model, Subaru’s Factory Service Manual (2012MY Impreza, Cooling/Heater System section) specifies a separate water inlet—commonly called the thermostat housing—that bolts to the water pump and retains the thermostat and seal. Subaru’s FAST parts catalogue for the 2012 Impreza (GJ/GP) also lists this water inlet/thermostat housing as a distinct component, confirming it’s a serviceable part rather than an internal casting.

The housing’s job is simple but critical: it anchors the thermostat at the engine’s lower radiator hose connection, directing coolant flow so the engine warms up briskly and then holds steady operating temperature. By opening only when the coolant reaches a set temperature, the thermostat and housing combo helps fuel efficiency, heater performance and engine longevity. The FB engine places the thermostat low in the cooling circuit, so the housing is easy to access under the front of the car once the undertray is off.

As part of regular servicing, it’s smart to inspect the housing and the area around the lower radiator hose. Look for pink or white crusting from dried coolant, seepage around the O-ring/gasket, and stains on the undertray. The plastic housing can warp with age and heat cycling, and the O-ring hardens over time. Many techs in Australia and New Zealand replace the thermostat, housing and seal as a set when chasing overheating or chronic leaks, or whenever the cooling system is overhauled.

• Common symptoms: slow warm-up, temperature swings, low heater output, coolant drips near the lower hose, or a sweet smell after a run.
• Good practice: use a new thermostat with the correct temperature rating, a fresh O-ring/seal, and fresh clamps if the spring clamp has lost tension.

Replacement pointers: drain the coolant cleanly, remove the lower hose and two housing bolts, and note the thermostat orientation (jiggle pin/air bleed at the top if applicable). Clean mating faces, lightly lube the O-ring with coolant, refit, and tighten the housing bolts to the factory torque (check the FSM—these small bolts are typically in the single-digit N·m range). Refill with Subaru blue long-life premixed coolant, bleed with the heater on hot, and squeeze the upper hose to purge air. After a decent test drive, recheck the level under the bonnet and inspect for weeps. Following the factory service information ensures the FB20 keeps its cool on Aussie and Kiwi roads.

FAQs

Where is the thermostat housing on a 2012 Subaru Impreza?
It’s mounted at the front lower side of the engine, where the lower radiator hose meets the water pump. Remove the undertray and you’ll see the plastic water inlet with two bolts securing it to the pump body.

Should the housing be replaced with the thermostat?
If there’s any warping, cracking, or pitting on the sealing face—or if leaks persist after a new O-ring—replace the housing with the thermostat. Many workshops fit both together to avoid repeat jobs.

What coolant should be used after replacing the housing?
Use Subaru-approved blue long-life premixed coolant. It’s formulated for aluminium engines and the FB cooling system. Always bleed the system properly to prevent air pockets and temperature fluctuations.