Parts for your 2019 Subaru Impreza-Thermostat

2019 Subaru Impreza Thermostat — what it does and when to replace it

Based on technical references including the Subaru Factory Service Manual for the 2019 model year Impreza (FB20 engine, cooling section) and the Subaru global parts catalogue for the GK/GT platform, this vehicle is fitted with an engine coolant thermostat. It’s mounted at the water pump/lower radiator hose housing and is specified to begin opening around the high‑80s °C. So yes, a thermostat is absolutely relevant on the 2019 Subaru Impreza.

The thermostat’s job is simple but critical: help the engine warm up quickly, then keep it sitting in the sweet spot for temperature under all conditions. By holding coolant back when the engine is cold, it gets the car up to operating temp faster for better fuel economy, lower emissions and strong cabin heat. Once warm, it meters flow to the radiator so the FB20 stays around the low‑90s °C, protecting the head gaskets and preventing hot spots that can shorten engine life.

There’s no fixed replacement interval for the thermostat, it’s typically replaced when it fails, when the water pump is done, or during major cooling system service. Common signs it’s on the way out include a temp gauge that runs low on the motorway but creeps up in traffic, slow warm‑up, weak heater after several minutes of driving, or a Check Engine Light with code P0128 (coolant temp below thermostat regulating temperature). Visible crusting around the housing or a swollen lower hose can also hint at issues.

When fitting a new unit, use a quality, engine‑correct thermostat and a fresh O‑ring or gasket. On the FB20, orient the jiggle pin/air bleed at 12 o’clock, clean the mating surfaces, and torque the housing bolts to the factory spec from the service manual. Refill with Subaru‑approved long‑life coolant (50/50 mix if not premixed), then bleed the system thoroughly—boxer engines can trap air. A spill‑free funnel, nose‑up parking and a warm‑up with the heater on can help purge bubbles. After the test drive, recheck the coolant level once it cools and look for leaks.

• Replace the thermostat proactively with the water pump or during a full cooling system refresh.
• Always install a new seal, and avoid mixing coolant types.
• Dispose of old coolant responsibly, it’s toxic to pets and wildlife.

Popular questions about the 2019 Subaru Impreza thermostat

Where is the thermostat on a 2019 Impreza?
It’s housed at the lower radiator hose connection on the engine, integrated with the water pump outlet. Access is from underneath or through the front once the splash shield is off. Expect a small coolant spill when removing the housing, so have a drain pan ready.

What temperature does it open?
The factory spec calls for an opening temperature in the high‑80s °C, fully open in the mid‑90s °C range. That keeps the FB20 engine near optimal operating temperature for efficiency and performance in Aussie and Kiwi climates.

Should it be replaced during a coolant change?
Not automatically. If the thermostat is original and the vehicle has high kilometres, or there are symptoms like P0128, fluctuating temps, or sluggish warm‑up, pairing a thermostat with a coolant service is smart. Otherwise, leave it until there’s a fault or you’re doing related work like a water pump.