Parts for your 2006 Subaru Impreza-Thermostat housing

2006 Subaru Impreza Thermostat Housing: What It Does and When To Replace It

Based on Subaru’s 2006 Impreza Factory Service Manual (Engine Cooling section) and the Subaru Electronic Parts Catalogue for EJ-series engines, the 2006 Subaru Impreza is fitted with a thermostat housing. Subaru identifies it as the water inlet/thermostat cover, mounted to the water pump where the lower radiator hose connects. So yes—this model absolutely uses a thermostat housing, and it’s a key part of the cooling system.

The thermostat housing’s job is to secure the thermostat, seal the coolant path, and route coolant from the lower radiator hose into the engine. On the EJ20/EJ25 engines common to the 2006 Impreza range, the housing clamps the thermostat in place with an O-ring, keeping everything sealed while coolant flow is regulated to get the engine up to temperature quickly and hold it steady on the move. A tidy housing and fresh seal help prevent leaks, overheating, and those annoying coolant smells under the bonnet.

Thermostat housing service isn’t usually a regular interval item, but it’s smart to inspect it whenever coolant is changed or the thermostat is replaced. Aluminium housings can pit or corrode at the hose neck, and the O-ring can flatten with age. If there’s chalky residue, staining, or dampness around the housing, it’s time for attention.

When replacing, a technician will typically drain the coolant, remove the lower radiator hose, then unbolt the housing from the water pump. The thermostat orientation is matched to the manual, a new Subaru-spec thermostat and O-ring are fitted, and the housing is refitted and tightened to factory torque. After refilling with the correct Subaru-approved coolant, the system is bled of air and checked for leaks at operating temperature. Avoid over-tightening—those alloy threads don’t like a gorilla with a spanner.

• Watch for: coolant weep at the housing seam, cracks, warped sealing face, or crusty white/green deposits.
• Best practice: replace the O-ring any time the housing is opened