Parts for your 2018 Subaru Forester-Thermostat

2018 Subaru Forester Thermostat – purpose, fitment and service advice

Technical sources confirm the 2018 Subaru Forester is fitted with an engine coolant thermostat. The Subaru Factory Service Manual for the Forester SJ (Cooling section) and the Subaru genuine parts catalogue list a wax‑pellet thermostat located at the water pump inlet on the lower radiator hose. Major aftermarket catalogues used by workshops in AU/NZ (e.g., Tridon, Gates) also list direct‑fit thermostats for the 2018 Forester (FB25 and FA20DIT), confirming factory fitment.

The thermostat’s job is simple but critical: it controls coolant flow so the engine warms up quickly and then stays in its sweet spot—roughly the low‑90s °C—regardless of whether it’s a frosty Dunedin morning or a hot day in Cairns. By holding coolant back when the engine is cold, it shortens warm‑up, improves fuel economy, reduces wear, and gives faster cabin heat. Once up to temp, it meters flow through the radiator to prevent overheating and keep emissions in check.

For servicing a 2018 Forester, the thermostat isn’t a routine replacement item like oil or filters, but it should be inspected whenever the coolant is changed or if there are temperature concerns. Many owners choose to replace it proactively during major cooling‑system work (such as a water pump service) or at higher kilometres to avoid surprises. Always use an OEM‑quality thermostat and a new O‑ring seal. Under the bonnet, access is typically from the front underside—remove the splash shield, drain enough coolant to drop the level, then pop the water inlet off the pump, swap the thermostat, and refit. Clean mating faces, seat the O‑ring properly, and tighten fasteners to the factory torque spec.

Refill with Subaru‑approved long‑life coolant (the blue Subaru Super Coolant is common in AU/NZ) at the correct mix, then bleed the system thoroughly. Park nose‑up if possible, set the heater to full hot, and run the engine while topping up to purge air. A spill‑free funnel makes life easier. Check for leaks, verify fan operation, and confirm stable temperature on a proper road test. Don’t run the Forester without a thermostat—doing so can cause slow warm‑up, poor heater performance, increased fuel use, and even overheating due to uncontrolled flow paths.

• Typical warning signs: slow warm‑up or running too cold, overheating, fluctuating gauge, weak cabin heat, or a P0128 fault code.
• Good practice: replace the thermostat and cap together if there’s any doubt, and always dispose of old coolant responsibly.

Popular questions about the 2018 Subaru Forester thermostat

Where is the thermostat on a 2018 Subaru Forester?
It sits at the water pump inlet on the lower radiator hose side, towards the front of the engine. Most techs access it from underneath after removing the front splash shield. Turbo and non‑turbo models share a similar location.

When should it be replaced?
Replace it if there are temperature control issues, during major cooling‑system work, or as preventative maintenance at higher kilometres. If the gauge wanders, the heater is weak, or a P0128 appears, it’s time for testing and likely replacement.

Can you drive a Forester without a thermostat?
No. Running without one leads to long warm‑up, poor fuel economy, potential engine wear, and paradoxically can still cause overheating. Always refit a correct‑spec thermostat with a new seal.