Parts for your 2021 Subaru Impreza-Thermostat

2021 Subaru Impreza Thermostat — Purpose, Service and Replacement

Based on Subaru’s Factory Service Manual for the 2021 Impreza (Cooling section) and the OEM parts catalogue for the FB20D engine, this model absolutely uses a conventional engine coolant thermostat mounted in the water pump/outlet housing, so the thermostat is both relevant and serviceable on the 2021 Subaru Impreza.

The thermostat’s job is simple but critical: it holds coolant flow closed until the engine reaches operating temperature, then meters flow to the radiator to keep the temperature steady. That quick, controlled warm‑up improves fuel economy, cabin heater performance, oil life and emissions, while the stable operating range protects the alloy block and heads from thermal stress.

Subaru doesn’t list a fixed change interval for the thermostat, it’s typically replaced on condition or during bigger cooling‑system jobs. If the vehicle shows overheating at speed, stubbornly low temperature on the gauge, or temperature swings in traffic, the thermostat is a prime suspect. When replacing, use a genuine or high‑quality OEM‑spec thermostat, install a new O‑ring/gasket, and refill with Subaru Super Coolant (blue) or an equivalent long‑life premix that meets the spec. Bleed the system carefully, set the heater to full hot, run the engine until the fans cycle, and top up the reservoir after a cold recheck. Always tighten fasteners to the factory torque values from the manual.

Common signs the thermostat needs attention include:

• Slow warm‑up, poor heater output, or a P0128 code for coolant temperature below thermostat regulating temperature.
• Temperature gauge wandering or spiking, especially after motorway climbs or stop‑start traffic.
• Overheating under load but cooling down at idle, or vice‑versa.
• Discoloured coolant, debris lodged at the thermostat, or repeated air in the system.

On these FB‑series engines the thermostat sits at the lower radiator hose on the water pump outlet, so expect a decent coolant dump, a drain pan and new hose clamps are smart. Most workshops allow about an hour or two. Pair the job with a new radiator cap and a close look at hoses, the water pump and the belt drive. Keeping records of coolant changes and any temperature‑related work helps with warranty conversations and resale.

Where is the thermostat on a 2021 Subaru Impreza?
It’s located at the lower radiator hose on the front of the engine, built into the water pump outlet housing. Access is usually from underneath after removing the undertray. Be ready to catch coolant, and ensure the new O‑ring seats cleanly in the housing groove.

What temperature does the 2021 Impreza thermostat open at?
The factory thermostat for the FB20D typically begins to open in the high‑80s °C and is fully open by the mid‑90s °C. The exact figure can vary by part revision, check the specs on the replacement thermostat or the service manual. If the engine runs too cool, the ECU may log a P0128 code.

Should the thermostat be replaced when changing coolant?
It isn’t mandatory, but it’s sensible if the car is older, high‑kilometre, or showing any temperature quirks. Many owners choose to replace the thermostat proactively around 8–10 years or 160,000–200,000 km. Always fit a new O‑ring/gasket and verify proper bleeding after the coolant service.