Parts for your 2015 Subaru Impreza-Thermostat

2015 Subaru Impreza Thermostat — What it does, and when to sort it

Based on technical references including the Subaru Factory Service Manual for the 2015 model year Impreza (FB-series engine, Cooling section) and Subaru’s genuine parts catalogue for the FB20 engine, the 2015 Subaru Impreza is fitted with an engine coolant thermostat. Repair manuals covering 2012–2016 Impreza models also specify the thermostat’s regulating temperature range and testing procedures, confirming it’s a standard, serviceable part of the cooling system.

The thermostat’s job is simple but vital: help the engine warm up quickly, then hold it at the right operating temperature. It stays closed when the engine’s cold to speed warm-up (better fuel economy and lower wear), then opens around its rated temperature to send coolant through the radiator. On the 2015 Impreza’s FB20, the thermostat is mounted near the water pump at the lower radiator hose. When it’s healthy, owners should see stable temperature readings and consistent heater performance.

While it isn’t a “replace on schedule” item, the thermostat deserves attention during routine servicing. A sluggish warm-up, fluctuating temp gauge, weak cabin heat at speed, or an overheating event under load are all red flags. Modern cars will often log a P0128 code when the thermostat sticks open and the engine runs too cool. Any coolant leaks around the housing, crusty residue, or distorted seals are also prompts to act.

• Check: temperature stability, heater output, and for coolant leaks at the housing and hoses.
• Replace if: overheating, slow warm-up, temp swings, P0128, or visible leakage/corrosion are present.
• Best practice: use an OE-spec thermostat and new gasket/seal, refill with Subaru-approved long‑life (blue) coolant and bleed air properly.

Replacement on the Impreza is straightforward for a competent home mechanic: drain enough coolant to get below the housing, remove the lower hose and housing, note the thermostat’s orientation, install the new unit with a fresh seal, then refill and bleed. Given the FB engines can trap air if rushed, many owners prefer a vacuum fill tool or a methodical bleed with the nose slightly raised and the heater on hot. After the job, a short drive with a scan tool watching coolant temperature can confirm the thermostat is regulating properly.

Look after this little valve and the Impreza runs at its sweet spot—better economy, stronger heater performance in winter, and less chance of head gasket or radiator headaches down the track.

FAQs

Does the 2015 Subaru Impreza have a thermostat, and where is it?
Yes. Technical documentation for the FB20 engine shows a conventional wax‑type thermostat. It’s located at the engine end of the lower radiator hose, in a housing near the water pump.

What are common signs the thermostat needs replacing?
Slow warm‑up, temperature gauge wandering, weak heater output at highway speeds, overheating under load, or a stored P0128 code are typical. Any leaks or corrosion at the thermostat housing are further clues.

Should it be replaced during a coolant change?
It isn’t mandatory, but if the vehicle has high kilometres, shows any symptoms, or the housing seal looks tired, it’s smart to replace the thermostat and seal while the system is drained. Always use OE‑spec parts and fresh coolant.