Parts for your 2015 Subaru Impreza-Radiator cap

2015 Subaru Impreza radiator cap — what it does, where it lives, and when to replace it

Technical reference check: Subaru documents confirm the 2015 Impreza (FB20 engine, GP/GJ series) does use a radiator cap. The Subaru Workshop/Service Manual for 2015MY Impreza (Engine: Cooling) specifies a pressure cap and testing procedure, the Subaru Genuine Parts Catalogue lists a “Cap–Radiator” (commonly 45137AE003, approx. 108 kPa/1.1 bar), and the 2015 Owner’s Manual warns to only remove the cap when the engine is cold. On this model the pressure cap is fitted to the pressurised coolant filler/header tank near the top radiator hose, not on the plastic radiator top tank itself—so a radiator cap is very much relevant and used.

On the 2015 Subaru Impreza, the radiator cap quietly does a power of work. It seals the cooling system, raises the boiling point of the coolant by holding pressure, and manages thermal expansion by allowing hot coolant to flow to the overflow bottle and then drawing it back as things cool down. Subaru’s FB-series layout places the cap on a pressurised header tank for efficient self-bleeding and consistent coolant circulation—handy under tough Aussie and Kiwi conditions.

As part of routine servicing, the cap deserves a quick once-over. A tired cap can cause hard-to-pinpoint issues like slow coolant loss, temperature swings, collapsed hoses, or an overflow bottle that either overfills or never seems to draw back. A workshop can pressure-test the cap to its rated spec (around 1.1 bar for the genuine item) and replace it if it won’t hold steady. Many techs treat the cap as a low-cost insurance item every 3–5 years, or sooner if inspection or testing says it’s time.

• Always check or remove the cap only when the engine is stone cold.
• Inspect the rubber seal for nicks, flattening, or crusty deposits.
• Look for staining around the neck of the filler tank—often a clue to seepage.
• If replacing, use a quality cap matching Subaru’s pressure rating and neck depth.

If the cap’s been off, top up with the correct Subaru-compatible long-life coolant mix and bleed per the service procedure so there’s no trapped air under the bonnet. Done right, the little cap keeps the Impreza’s temperatures tidy, the heater hot, and the head gaskets happy.

• Common signs it’s failing: random overheating, weak cabin heat at idle, visible coolant crust near the cap, or a hose that goes pancake-flat after cool-down.

Popular questions

Where is the radiator cap on a 2015 Subaru Impreza?
It’s on the pressurised coolant filler/header tank mounted high in the engine bay, not on the radiator itself. This location helps purge air and maintain consistent coolant flow.

What pressure cap does a 2015 Impreza use?
Most NZ/AU 2015 Impreza FB20 models specify a cap around 108 kPa (1.1 bar), as per the genuine Subaru cap. Always confirm by VIN or the label on the original cap before purchase.

How often should the radiator cap be replaced?
There’s no strict interval in the owner’s manual. Have it inspected and pressure-tested at regular services, many workshops replace it every 3–5 years or at the first sign of leakage or test failure.