Parts for your 2015 Subaru Legacy-Spark plugs

2015 Subaru Legacy (Liberty) spark plugs — what they do and when to replace them

Yes, the 2015 Subaru Legacy (known as Liberty in Australia) uses spark plugs. Technical references that confirm this include the 2015 Subaru Legacy/Liberty Owner’s Manual and maintenance schedule, Subaru global parts catalogues listing plug part numbers for the FB25 2.5‑litre and EZ36 3.6‑litre petrol boxer engines, and application guides from OEM plug makers such as NGK and Denso. Those sources specify iridium spark plugs and list spark plug replacement as a scheduled service item. Diesel boxer engines (which use glow plugs, not spark plugs) were not the common AU/NZ fitment for MY15 Legacy/Liberty.

Spark plugs ignite the compressed air–fuel mix in each cylinder, kicking off the combustion that drives the pistons. In the 2015 Legacy’s flat‑four and flat‑six petrol engines, a coil‑on‑plug system sends precise voltage to long‑life iridium plugs. Strong, reliable spark keeps cold starts crisp, fuel economy steady, emissions clean and throttle response sharp.

Subaru’s maintenance literature for MY15 lists spark plugs as a periodic replacement item, typically around 96,000 km (60,000 miles), with earlier replacement if symptoms appear or the car sees harsh, short‑trip, dusty, or high‑load use. Owners should always follow the exact interval shown in the Owner’s Manual and the service schedule sticker under the bonnet.

• Likely service interval: about 96,000 km (60,000 miles) for iridium plugs on FB25 and EZ36 engines.
• Common signs it’s time: rough idle, hesitation or misfire under load, higher fuel use, sluggish starts, or a flashing MIL under acceleration.

When replacing, use the correct iridium specification (Subaru‑approved NGK or Denso equivalents). Iridium fine‑tip plugs are pre‑gapped, the gap is typically around 1.1 mm, and should not be forced open or closed. Fit to clean, cool aluminium heads using a torque wrench and the book spec (usually in the 18–22 N·m range). Modern plated‑thread plugs generally don’t need anti‑seize, applying it can alter torque and risk over‑tightening unless the plug maker specifically permits it. A light smear of dielectric grease on coil boots helps future removal and keeps moisture out.

Staying on top of spark plug condition pays off with smoother running, stronger mid‑range pull and better fuel economy. For a 2015 Legacy/Liberty, it’s an inexpensive bit of maintenance that keeps the boxer humming along nicely.

Popular questions about 2015 Subaru Legacy spark plugs

What spark plug type and gap does a 2015 Subaru Legacy use?
Subaru specifies long‑life iridium spark plugs (NGK or Denso OEM equivalents) for both the 2.5‑litre FB25 and 3.6‑litre EZ36 petrol engines. The gap is factory‑set, don’t adjust iridium fine‑tip plugs. Expect about 1.1 mm, but the exact spec is on the under‑bonnet label and in the Owner’s Manual.

How often should spark plugs be replaced on a 2015 Legacy/Liberty?
Subaru’s service schedule calls for replacement at roughly 96,000 km (60,000 miles). Cars that tow, run lots of short trips, or operate in dusty or very hot conditions may benefit from earlier checks. Always follow the interval printed in the manual for the specific engine.

Can iridium spark plugs be cleaned and reused?
Abrasive cleaning or wire‑brushing can damage the fine iridium tip, and re‑gapping can break or bend the precious‑metal electrode. If a plug is fouled or worn, replacement with the correct OEM‑equivalent part is the safe move.