Parts for your 2017 Subaru Impreza-Thermostat housing

2017 Subaru Impreza thermostat housing — what it does and how to look after it

For the 2017 Subaru Impreza fitted with the FB20D 2.0‑litre boxer engine, a thermostat housing is absolutely used and serviceable. Subaru’s factory service information for the 2017MY Impreza (FB-series Cooling section) and the Subaru FAST/OEM parts catalogue both show a conventional wax‑pellet thermostat mounted in the water pump and retained by a cast alloy housing (often listed as the water inlet/thermostat cover) at the lower radiator hose connection. So, yes — this model has a thermostat housing, and it matters.

The thermostat housing’s job is to locate and seal the thermostat, route coolant from the lower radiator hose into the pump, and keep everything leak‑free under pressure and temperature swings. Paired with the thermostat, it helps the engine warm up promptly and then hold a stable operating temperature, which protects fuel economy, performance, and engine longevity.

There’s no fixed replacement interval for the housing itself, but it should be inspected any time the cooling system is serviced. Typical triggers for attention include age‑hardening of the O‑ring, corrosion/pitting on the housing’s sealing face, or damage from past overtightening. When the thermostat is replaced, the housing comes off — that’s the perfect time to renew the O‑ring and check the mating surfaces.

• Common symptoms to watch: coolant weep or crust around the lower hose flange, slow warm‑up or over‑cooling (stuck‑open stat), overheating (stuck‑closed stat), and temperature swings at highway speeds.
• Recommended practice: use a genuine or high‑quality thermostat and O‑ring, clean the sealing faces, and torque the housing bolts to factory spec. Avoid RTV unless the service manual explicitly calls for it.

On an FB‑series Impreza, a thermostat/housing job pairs nicely with a coolant service. Use the correct Subaru long‑life blue coolant (or an approved equivalent), mix as specified if not pre‑mixed, and bleed air properly with the heater on. After refilling, let the fans cycle, check for leaks under the bonnet and from underneath, and top up the reservoir after a short drive. If the housing shows heavy pitting, cracks, or a warped flange, replace it rather than trying to nurse it along