How often should I replace spark plugs on my Subaru Forester?

Most newer Forester models use long-life iridium or platinum plugs with a replacement interval around 105,000 miles (about 168,000 km). If your Forester uses copper plugs, the interval is shorter, typically around 60,000 miles (96,000 km). Always confirm the exact interval in your owner's manual or with a Subaru technician, since model year and engine choice matter.

Engine-specific guidance

Forester engines vary by generation and market. The following guidelines reflect typical factory recommendations for recent model years, but your exact figure may differ. Check the manual for your VIN to be sure.

2.5L naturally aspirated engines (FB25)

Most FB25 Foresters use iridium or platinum plugs with a replacement interval near 105,000 miles (approximately 168,000 km). Some early or regional variants may still use copper plugs with shorter intervals (about 60,000 miles). Use the plug type specified in the owner’s manual.

2.0L turbo engines (FA20/FB20, often in XT trim)

Turbos can run hotter and place more stress on plugs. Subaru generally lists the same long-life replacement window (roughly 100,000–105,000 miles) if iridium/platinum plugs are used. If your car uses older copper plugs, expect a shorter interval and plan for more frequent inspection.

In all cases, if you tow frequently, drive in heavy stop-and-go traffic, or make many cold starts, you may benefit from inspecting earlier and replacing within the window if signs of wear appear.

When to replace spark plugs: signs you should watch for

Even with a long-life maintenance interval, plugs can wear or foul earlier depending on conditions. Look for these warning signs as your Forester racks up miles.

• Rough idle or engine misfires, especially at idle or low speed


• Decreased acceleration or power, particularly under load or boost


• Hard starting or longer cranking to start the engine


• Poor fuel economy or increased emissions


• Visible signs of fouling, oil deposits on plugs, or damage to spark plug boots

If you notice any of these symptoms, have the ignition system and fuel system checked. Replacing plugs is often part of a broader service, especially if wires or coils show wear.

DIY vs professional replacement: considerations

Replacing spark plugs on a Subaru Forester is a common DIY job for many enthusiasts, but it requires the right tools and care to avoid cross-threading or damaging ignition coils. Consider these factors before you start.

• Accessibility: Four-cylinder Foresters (common) provide straightforward access to the spark plugs; a V6 or engine with coil-on-plug designs may be similarly accessible, depending on generation.


• Tools and safety: You’ll need a spark plug socket, extension, ratchet, torque wrench, and possibly a compression/plug-gap gauge. Do not exceed recommended torque; apply dielectric grease to boots, and avoid anti-seize on modern iridium/platinum plugs unless specified.


• Plug type and gap: Use the exact plug type and gap spec listed for your engine. Substituting may lead to misfires or reduced performance.


• Cost vs time: Parts cost is modest, but labor can add up if you’re paying shop rates. A DIY replacement saves money but requires care.

If you’re unsure or uncomfortable, a trained technician can ensure the plugs are torqued correctly and that any coils or wires are not damaged during the process.

Summary

For most Subaru Forester models, replace spark plugs roughly every 100,000–105,000 miles when you use long-life iridium or platinum plugs, and about 60,000 miles if copper plugs are still fitted. Always verify intervals in the owner’s manual for your exact engine and year. Stay alert for symptoms like rough idle, reduced power, or poor fuel economy, and plan replacements as part of routine maintenance to maintain performance and efficiency.

Why are Subaru spark plugs so expensive?

Subaru spark plugs are expensive due to the significant labor involved in replacing them and the cost of the specialized, long-life iridium spark plugs used in most models. The unique boxer engine configuration places the spark plugs on the sides, requiring mechanics to lift the engine or partially disassemble other components to access them. This difficulty and the use of high-quality parts drive up the total cost.
 
Labor costs

• Difficult access: The boxer engine's flat, opposed cylinders mean spark plugs are located low down near the frame rails, unlike many inline engines where they are on top.
• Time-consuming replacement: Accessing the plugs can require significant labor, such as unbolting the engine from its mounts and carefully lifting it slightly to create enough space.
• Increased hourly rates: The extended time needed for the job results in higher overall labor costs from dealerships or independent mechanics who charge by the hour. 

Part costs

• Premium materials: Subaru uses premium iridium or platinum spark plugs, which are more expensive than standard copper plugs but offer a longer lifespan, often exceeding 100,000 miles. 
• Precise manufacturing: These plugs are manufactured with high precision to meet Subaru's specific engine requirements, which adds to their cost. 

Other factors

• Bundled services: Dealerships sometimes bundle spark plug replacement with other maintenance tasks for the 60,000-mile service, making the final bill appear higher. 
• Dealer markup: While the parts themselves are more costly, the primary reason for the high price is the labor involved, and dealership labor rates are often higher than those at independent shops. 

Do I really need to change my spark plugs at 100,000 miles?

Yes, replacing spark plugs at 100,000 miles is a common and often necessary maintenance task, as many modern vehicles have spark plugs rated to last that long. However, the exact interval depends on your vehicle's make, model, and the type of spark plug installed, so you should always check your owner's manual for the manufacturer's specific recommendation. 
This video explains when to change spark plugs and what happens if you don't: 1mHonest Mechanic ColoradoYouTube · Dec 24, 2023
Why it's important

• Engine performance: Spark plugs that are past their prime can cause rough idling, poor acceleration, and decreased fuel efficiency. 
• Preventing damage: Over time, worn-out plugs can put extra stress on other components, like ignition coils, and may seize in the engine block, making them harder to remove later. 
• Preventative maintenance: Replacing spark plugs is a form of preventative maintenance that can prevent more costly repairs down the line. 

This video explains why it's important to change spark plugs at 100k miles: 52sWhite One Auto, LLCYouTube · May 19, 2021
What to consider

• Owner's manual: Your vehicle's owner's manual is the most reliable source for the correct replacement interval and recommended plug type. 
• Spark plug type: The lifespan of spark plugs varies by material. Iridium plugs are often rated for 100,000 miles or more, while other types like copper or platinum have shorter lifespans. 
• Driving conditions: Severe driving conditions can shorten the life of spark plugs, so your interval may be shorter than recommended if you frequently tow, drive in extreme temperatures, or frequently start and stop. 

How long do spark plugs last on a Subaru Forester?

Maintenance Tips for Subaru Spark Plugs
To extend the life of your Subaru spark plugs and ensure your engine runs smoothly, regular maintenance is key. This includes following Subaru's recommended service intervals—typically every 60,000 miles for standard replacements.

How do I know if my Subaru spark plugs are bad?

One of the first signs of spark plug wear is trouble starting your Subaru. If the engine cranks longer than usual before starting or if you experience intermittent starting issues, it's time to inspect your spark plugs.