Parts for your 2018 Subaru Legacy-Ignition coils

2018 Subaru Legacy ignition coils – what they do and when to service them

Technical references confirm the 2018 Subaru Legacy runs coil-on-plug ignition coils and they are absolutely relevant to this model. Sources include the Subaru Tech Information System service manual for MY2018 Legacy/Outback (Engine: Ignition System, Diagnosis sections for DTCs P0351–P0356) and the Subaru Genuine Parts catalogue (FAST), both of which list individual ignition coils—four on the 2.5‑litre FB25 and six on the 3.6‑litre EZ36. These factory documents describe a direct ignition system with one coil per cylinder, controlled by the engine ECU.

On a 2018 Subaru Legacy, the ignition coils sit right on top of the spark plugs and turn the car’s 12‑volt supply into the big voltage needed to fire each plug cleanly. It’s a neat, efficient setup—no old‑school leads to chase, just compact coil-on-plug units timed by the ECU. The FB25 2.5i uses four coils, the 3.6R uses six. When they’re healthy, starts are crisp, fuel use stays tidy, and the engine pulls smoothly across the rev range.

Coils aren’t a routine service item like oil or filters, but they do benefit from a check whenever plugs are replaced (typically around 100,000 km, depending on market and plug type). During servicing, a tech will:

• Inspect coil boots for cracks, hardening, or tracking marks.
• Check for oil in plug tubes (a sign of a cam cover seal leak that can kill coils).
• Verify tight, clean electrical connectors and no green corrosion.
• Use a dab of dielectric grease inside the boot to help sealing and future removal.

Typical warning signs a coil is on the way out include a rough idle, hesitation under load, higher fuel use, hard starting, and a check engine light with codes like P030X (misfire) or P035X (ignition coil circuit). If a single cylinder is acting up, replacing just the faulty coil is fine after confirming the spark plug and injector are OK. If several coils are aging on a higher‑kilometre car, doing them as a group can save repeat labour.

DIY? Disconnect the battery, blow debris out of plug wells, and seat the new coil fully on the plug. Avoid using a pressure washer in the engine bay—water in the plug tubes can trigger misfires. On the 3.6R, access to the rear cylinders is tighter, many owners prefer a workshop to handle those. Stick with quality OEM‑spec coils (often Denso for Subaru) to keep the Legacy running sweet under the bonnet.

• Popular questions about 2018 Subaru Legacy ignition coils

How long do the ignition coils last?
With good plugs and no oil or moisture in the plug tubes, coils commonly last well past 150,000 km. There’s no fixed replacement interval—inspect them during major services and act on any misfire or cracking found.

Can just one coil be replaced, or should they all be done?
It’s perfectly acceptable to replace a single failed coil once the fault is verified. If multiple coils show age‑related issues on a high‑kilometre car, replacing them as a set can be smarter for long‑term reliability.

Which engines in 2018 Legacy use coils, and how many?
All 2018 Legacy petrol engines use coil-on-plug ignition: the 2.5i (FB25) has four coils—one per cylinder, the 3.6R (EZ36) has six. Part numbers vary by VIN, using OEM‑spec coils ensures correct fit and performance.