Parts for your 2017 Subaru Impreza-Thermostat

2017 Subaru Impreza Thermostat — Purpose and Service Advice

Based on Subaru’s factory service literature for the 2017MY Impreza (FB20 engine) and OEM parts catalogue listings available via Subaru’s Technical Information System, this vehicle absolutely uses a conventional engine coolant thermostat. It’s a wax‑pellet style unit mounted at the water pump inlet on the lower radiator hose side, and it’s serviced as a separate component with an O‑ring seal.

The thermostat’s job is straightforward but vital: it controls coolant flow so the FB20 warms up quickly and then stays in its sweet spot under all sorts of driving. When the engine’s cold, the thermostat stays shut to help it reach operating temperature faster—better fuel economy, smoother running, and less wear. Once it’s up to temp, the thermostat opens progressively, sending coolant through the radiator to keep everything stable, even on a scorching summer arvo or a long slog up the Kaimais.

As part of routine servicing on a 2017 Impreza, the thermostat isn’t typically replaced at every coolant change, but it should be inspected any time the cooling system is worked on. Many workshops treat it as preventative maintenance somewhere around the 8–10 year or 150–200,000 km mark, or sooner if there are symptoms of trouble. Subaru’s blue long‑life coolant is the go, mixing types or running old coolant can shorten thermostat life.

• Common signs it may be on the way out: slow warm‑up, temp gauge wandering, heater blowing cool at idle but hot on the move, fans running more than usual, or a CEL for coolant temp performance.
• Service tips: always fit a new genuine‑spec O‑ring, clean the mating surfaces, and align the jiggle valve as specified in the service manual. Refill with the correct pre‑mix and bleed the system thoroughly to avoid air pockets under the bonnet.
• Good practice while you’re there: check the lower hose condition, radiator cap rating, and for any crusty white residue that hints at minor leaks.

If the thermostat sticks shut, the engine can overheat quickly, if it sticks open, fuel use rises and the cabin heater underperforms. Either way, a fresh thermostat is a relatively low‑cost fix that protects the FB20 from bigger dramas down the track.

Where is the thermostat on a 2017 Subaru Impreza?

On the FB20 engine it’s housed at the water pump inlet, where the lower radiator hose connects to the pump. Access is from the front underside of the engine bay. A sealing O‑ring sits between the thermostat and the housing.

When should the thermostat be replaced?

It’s often replaced reactively if there are temp control issues, or preventatively around 8–10 years/150–200,000 km. Many techs renew it whenever there’s a major cooling system service, especially if the pump or hoses are being done.

What coolant should be used after thermostat replacement?

Use Subaru‑approved blue long‑life coolant (pre‑mixed to the correct ratio). Avoid mixing types. After filling, bleed the system properly and confirm stable operating temperature on a test drive.