Parts for your 2015 Subaru Forester-Thermostat

2015 Subaru Forester Thermostat

Per Subaru’s factory service manual for the SJ-series Forester (MY2015) and Subaru’s official parts catalogue, the 2015 Subaru Forester is fitted with an engine coolant thermostat across its petrol and diesel engines (FB25 2.5L, FA20F 2.0L turbo, and EE20 2.0L diesel). The unit sits at the water pump inlet on the lower radiator hose side, managing coolant flow to keep the engine at a stable operating temperature.

The thermostat’s job is simple but crucial: help the engine warm up quickly, then hold it right in the sweet spot for efficiency, performance, and longevity. Closed when cold, it routes coolant within the engine so it heats promptly, as temperature climbs, the valve opens—typically starting in the low 80s °C and fully open by the mid 90s °C—sending coolant through the radiator to shed heat. That stability protects head gaskets, reduces fuel use, improves heater performance under the bonnet’s skin, and keeps emissions in check.

On a 2015 Forester, a healthy thermostat means steady temperature on the gauge and reliable cabin heat. If it sticks open, the engine can take ages to warm up, fuel economy drops, and the heater feels weak. If it sticks closed, overheating can happen fast—never keep driving an overheating Forester, pull over safely and shut it down.

Thermostats aren’t a scheduled replacement item in Subaru servicing, but they’re fair game if symptoms appear or when major cooling work is being done (e.g., water pump or radiator replacement). It’s smart to pair a new radiator cap and fresh Subaru-spec long-life coolant at the same time. When replacing the thermostat:

• Use a quality OEM-spec thermostat and a new gasket/O-ring.
• Install with the jiggle pin/bleed hole oriented per the manual (usually at the top).
• Clean mating faces, don’t over-tighten the housing bolts.
• Refill with the correct silicate-free, long-life blue coolant and properly bleed air.
• After a road test, check for leaks and confirm stable temperature and strong heater output.

Warning signs that it’s time to investigate include fluctuating temp readings, slow warm-up, poor heater performance, coolant pushed into the overflow, or overheating. Catching a dodgy thermostat early can save a head gasket and a lot of coin.

Popular questions about the 2015 Subaru Forester thermostat

Where is the thermostat located on a 2015 Forester?
It’s mounted at the water pump inlet where the lower radiator hose meets the engine. Access is from underneath or from the front once the undertray is off, depending on engine. The housing contains the thermostat and seal.

How often should the thermostat be replaced?
There’s no fixed interval in Subaru’s schedule. Replace it if there are symptoms (overheating, slow warm-up, temp swings) or opportunistically during major cooling-system work. Always fit a new seal and fresh coolant when you do.

What are common signs of a failing thermostat?
A stuck-open stat gives slow warm-up and a lukewarm heater, a stuck-closed stat drives the temp up quickly. Other clues include unstable gauge readings and coolant being pushed into the overflow. If overheating starts, stop the vehicle and let it cool—don’t risk engine damage.