2021 Subaru Forester: Spark Plug Specifications

A complete guide to spark plug replacement for the 2021 Subaru Forester, with part numbers, torque specs, and essential tips.

Quick Reference

Why The Right Plug Matters

The humble spark plug plays a monumental role in your Forester's performance, efficiency, and reliability. It's not just a "one-size-fits-all" part. Using the correct plug ensures a precise, powerful spark at the exact right moment. The 2021 Forester's 2.5L direct-injection engine is designed for long-life iridium spark plugs.

• Heat Range: This is a spark plug's ability to dissipate heat. A plug that is too "hot" can overheat and cause pre-ignition (detonation), potentially damaging the engine. A plug that is too "cold" can run too cool to burn off carbon deposits, leading to fouling and misfires. The OEM-specified plug has the correct heat range for your engine under all operating conditions.
• Electrode Material:
  • Iridium: The electrode material specified for your Forester. Iridium has an extremely high melting point and is incredibly hard, allowing for a very fine-wire center electrode. This requires less voltage to create a spark, provides a more focused and consistent spark, and offers an exceptionally long service life—typically 60,000 to 100,000 miles.
  • Platinum: A step down from iridium, platinum plugs were the long-life standard for many years. They offer good performance and longevity (around 60,000 miles) but can't match the durability and fine-wire performance of iridium.
  • Copper: Basic copper plugs are excellent conductors but have a very short lifespan, often needing replacement every 20,000-30,000 miles. They are not recommended for modern engines like the one in your Forester, which are designed for the high-performance and long-life characteristics of iridium.

How to Gap a Spark Plug

Spark plug gap is the distance between the center and ground electrodes. If this gap is wrong, the spark can be weak or inconsistent, leading to poor performance and fuel economy. Iridium plugs like the NGK units for your Forester come pre-gapped from the factory (at 1.1mm / 0.043"). You should always verify the gap before installation.

Use a feeler gauge or a specific wire-style spark plug gapper. Never use the cheap "coin-style" gappers, as they can put pressure on the fragile iridium center electrode and damage it. If an adjustment is needed, do not pry against the center electrode. Instead, gently bend the ground strap (the curved piece of metal over the top) by tapping it on a hard surface to close the gap or using the tool's lever to gently bend it open. The goal is to have the gauge slide in and out with a slight, smooth drag.

Replacement Procedure Tips

Replacing plugs on a Subaru boxer engine is a bit different than on an inline-four or V6 because the spark plugs are on the sides of the engine, facing the frame rails. The 2021 Forester uses a modern coil-on-plug ignition system, where each spark plug has its own ignition coil mounted directly on top of it. This eliminates the need for a distributor and spark plug wires.

1. Access: Patience is key. To get to the coils and plugs, you'll need to remove the air intake ducting and housing on the passenger side. On the driver's side, removing the battery is often necessary to create enough room to work.
2. Coil Removal: Once you have access, disconnect the electrical connector from the ignition coil. Remove the single bolt holding the coil in place. The coil should then pull straight out of the cylinder head with a gentle twist. A special spark plug socket with a rubber insert or magnet is essential to hold the plug and lift it out of the deeply recessed plug well.
3. The Anti-Seize Debate: This is critical. NGK, the OEM supplier, applies a special trivalent plating to its plug threads. This coating acts as its own release agent and prevents seizing. NGK explicitly advises against using additional anti-seize compound on these plugs. Doing so lubricates the threads, which can easily lead you to over-torque the spark plug, potentially stretching the plug shell or damaging the threads in the aluminum cylinder head.
4. Dielectric Grease: While anti-seize is a no-go, applying a small amount of dielectric grease to the inside of the ignition coil boot (where it contacts the spark plug's porcelain top) is highly recommended. This prevents voltage leaks, protects against moisture, and keeps the boot from getting stuck to the plug, making future replacements much easier.

Known Issues

Unlike some engines notorious for spark plug problems—such as the Ford 3-valve Triton engines where two-piece plugs would break off in the head—the Subaru FB25DE engine in your 2021 Forester has no such widespread, known issues. The primary "issue" is simply the challenging access due to the boxer engine's design. As long as you use the correct plug, do not use anti-seize, and torque the plugs to the proper specification (15.5 lb-ft / 21 Nm), you are unlikely to encounter any problems.

Need plugs replaced? Find a shop on Repairius.