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A misfire can destroy your catalytic converter in minutes. A misfire is a cylinder that fails to light its fuel. On a turbo Subaru it often ends in a converter bill over $1,000.

So do not guess on plugs. Use the factory plug: the NGK 6481 ILFR6B Laser Iridium plug, sold in a 5-piece pack for Subaru and Volvo applications. This guide puts the gap, the torque, and the interval in plain numbers.

Why this plug suits a turbo EJ

Boost is the extra pressure your turbo feeds the engine, and it makes every spark harder to fire. A fine-wire iridium center electrode fires at lower voltage, so it keeps its margin exactly where a turbo runs out of it. Iridium is a hard precious metal, and NGK laser-welds it into a thin wire.

Copper plugs are the common cheap alternative, and they erode within 15,000-20,000 miles of boost. The ILFR6B is rated for the 30,000-mile cycle. That is why Subaru specified iridium at the factory.

• Stock or lightly modified engine? Stay at the factory heat range 6 — the number that sets how hot the plug runs.
• Retuned for more boost? Your tuner's plug and gap advice comes first. Going colder is a tuning decision, not a default upgrade.
• Buying once? The 5-pack covers four plugs plus a spare.

Check the gap before anything goes in the engine

The gap is the small space the spark jumps across. ILFR6B ships pre-gapped at about 0.031 inch (0.8 mm) — the B in ILFR6B is that gap code. Check every plug anyway, even in a sealed NGK box.

1. Gap-check all four with a wire or coin gauge. The tool costs a few dollars. Plugs shipped in "11" packaging have measured 0.044" on arrival — a gap that wide misfires under load.
2. Bend only the ground electrode, and only gently. Never pry the iridium center electrode; it is thin by design. A bent center electrode is a scrap plug.
3. Set all four to the same number. One wide gap among three correct ones gives a single-cylinder misfire code that looks like a bad coil.

Working ranges worth knowing: stock engines run 0.028"-0.031". Cars tuned for higher boost usually close toward 0.028". Heavily modified setups on aftermarket ignition run 0.024"-0.028".

Full specifications at a glance

Worn plug symptoms, and what they imitate

Suspect worn plugs first when you see three things together: hesitation under boost, a rough cold idle that clears when warm, and a P0301-series code. A P0301 code is your car's way of saying cylinder 1 is misfiring. Humid weather and hard pulls make the symptoms arrive sooner.

The reason is simple: worn electrodes widen the gap as metal erodes away. A coil that fired a 0.031" gap easily strains with a 0.040" gap at 15 psi of boost.

• Same misfire, different part: a cracked coil boot misfires under load too. Move a suspect coil to another cylinder — the ignition coil diagnosis guide walks the swap test.
• If the misfire stays put: the suspect is the plug or the injector. A tired injector gives the same rough idle, and the 16611-AA720 fuel injector set for the WRX STi, Forester, and Impreza is the flow-matched fix.
• Choosing between brands: the aftermarket vs OEM spark plug comparison shows where the cheap plugs actually differ.

Replace all four, not one

Reaching the plugs is most of the labor, not the plugs themselves. Two plugs sit on top of each cylinder head, and the rear pair hides under the intake plumbing.

So buy the set and touch the engine once. The 5-piece pack exists for exactly this: four plugs plus a spare, or five for a 5-cylinder application.

While the intake is off, inspect the neighborhood:

• Look for oil in the plug wells. The plug tube seals in the valve cover gasket weep oil on high-mileage EJs.
• Oil kills coils. Oil-soaked boots cause the same misfire complaints as worn plugs.
• Need more ignition parts? Plugs, coils, wire sets, and sensors all live in the ignition systems collection.
• Refreshing a 6-cylinder Subaru in the same order? The pre-gapped NGK PFR5N-11 laser platinum set is the companion part.

Frequently asked questions