Last summer, a 2016 Subaru Forester owner came into our service center complaining that his vehicle felt sluggish during acceleration on the highway and fuel economy had dropped from 27 mpg to 23 mpg over several months. He assumed he needed major engine work because the performance loss had been gradual but noticeable. When we diagnosed the vehicle, we discovered the spark plugs had never been replaced at the recommended 60,000-mile interval, and at 97,000 miles they were severely worn with electrode gaps 50% wider than specification. After spark plug replacement for $285, his Forester's performance and fuel economy returned to normal immediately. Spark plug replacement at the 60,000-mile interval as specified? The same $285, but performed 37,000 miles earlier, preventing the extended period of reduced performance and wasted fuel that cost him approximately $350 in extra gas over those miles.

That Forester owner made a mistake common among Subaru owners: assuming that because the engine still runs, spark plug replacement can be postponed indefinitely. The reality is that spark plugs are precision wear items with specific service lives, and while your engine will continue operating with worn plugs, it does so with compromised performance, reduced efficiency, and increased risk of misfires that can damage expensive catalytic converters.

Subaru specifies spark plug replacement intervals based on extensive engineering testing to determine when plugs typically degrade beyond acceptable performance. For most modern Outback and Forester models, that interval is 60,000 miles or 6 years, whichever comes first. However, driving conditions in Southwest Ohio, with our temperature extremes, stop-and-go traffic on I-75 and I-675, and short-trip driving common around Dayton, can accelerate spark plug wear beyond what the standard interval assumes.

This guide explains exactly why spark plugs need replacement, what factors affect their lifespan in your Outback or Forester, how to recognize symptoms of worn plugs, and why adhering to replacement intervals protects both performance and expensive emission control components.

Understanding Spark Plug Function in Subaru Engines

Spark plugs create the electrical spark that ignites the air-fuel mixture in your engine's combustion chambers. This seemingly simple function is actually extremely demanding, as spark plugs must perform flawlessly thousands of times per minute while operating in temperatures exceeding 1,500 degrees Fahrenheit and pressures over 1,000 PSI.

Modern Subaru engines in Outback and Forester models use either the 2.5L naturally aspirated four-cylinder (FB25), the 2.4L turbocharged four-cylinder (FA24), or the 2.0L turbocharged four-cylinder (FA20) in older models. All of these engines use a flat-four (boxer) configuration where cylinders oppose each other horizontally, which creates unique challenges for spark plug access and replacement.

Each spark plug consists of several components: a center electrode that the electrical current jumps from, a ground electrode that the spark jumps to, a ceramic insulator that prevents electrical leakage, and metal threads that secure the plug in the cylinder head. The gap between the center and ground electrodes is precisely specified, typically 0.039-0.043 inches depending on the engine model.

When the ignition coil sends electrical current to the spark plug, the voltage builds until it overcomes the resistance created by the air gap between electrodes. At that moment, the spark jumps across the gap, igniting the compressed air-fuel mixture. This process repeats for every combustion cycle in that cylinder, which at 2,000 rpm means each spark plug fires 1,000 times per minute.

The extreme operating environment gradually erodes the electrodes. Each spark removes microscopic amounts of metal from both the center and ground electrodes, slowly widening the gap. As the gap increases, higher voltage is required to generate a spark, which stresses the ignition coil and can eventually lead to misfires when voltage requirements exceed what the coil can produce.

Subaru's Symmetrical All-Wheel Drive system and boxer engine configuration create constant, balanced loads on the engine that differ from front-wheel drive vehicles. While this doesn't directly affect spark plug wear rates, it means that any performance degradation from worn plugs affects the drivetrain's efficiency and the precision of the AWD system's power distribution.

"Many customers are surprised when I recommend spark plug replacement at 60,000 miles because the engine seems to run fine," says Jennifer Walsh, Service Manager at our Miamisburg Centerville Road location. "What they don't realize is that 'runs fine' is relative. Yes, the engine runs, but it's not running as efficiently as it should. Worn spark plugs with excessive gap require higher voltage, which can stress ignition coils. They create weaker sparks that don't ignite the air-fuel mixture as completely, reducing power and fuel economy. And they increase the risk of misfires that can damage catalytic converters costing $1,200-2,000 to replace. The $285 spark plug service protects against all of these problems while restoring the performance and efficiency the vehicle had when new."

Factory Spark Plug Replacement Intervals for Outback and Forester

Subaru specifies different spark plug replacement intervals depending on the model year and engine type. Understanding the correct interval for your specific vehicle ensures you maintain optimal performance and prevent problems.

2015-2024 Outback and Forester with 2.5L FB25 engine: Replace spark plugs every 60,000 miles or 6 years, whichever comes first. This engine uses iridium spark plugs that provide extended service life compared to traditional copper plugs but still require replacement at this interval.

2019-2024 Outback and Forester with 2.4L FA24 turbo engine: Replace spark plugs every 60,000 miles or 6 years, whichever comes first. Turbocharged engines create higher combustion pressures and temperatures that stress spark plugs, making adherence to the replacement interval particularly important.

2014-2018 Forester XT with 2.0L FA20 turbo engine: Replace spark plugs every 60,000 miles or 6 years, whichever comes first. This turbocharged engine uses premium iridium plugs designed for the higher stresses turbocharging creates.

Pre-2015 models with various engine configurations: Replacement intervals vary from 30,000-60,000 miles depending on specific engine and plug type. Consult your owner's manual or contact our service department with your VIN for the correct interval for older models.

The "whichever comes first" language is critical. Even if your Forester hasn't reached 60,000 miles, if it's been six years since the last spark plug service, time-based replacement is necessary. Spark plugs degrade from age regardless of mileage due to thermal cycling, exposure to combustion byproducts, and electrode erosion that occurs even during relatively limited use.

Severe service conditions reduce these intervals. Subaru defines severe service as: frequent trips of less than 5 miles; extensive idling or stop-and-go driving; driving in dusty conditions; towing or hauling; or operation in temperature extremes. Many Dayton-area drivers meet these criteria through normal commuting patterns, suggesting spark plug replacement every 45,000-50,000 miles rather than waiting for the 60,000-mile maximum.

The cost of spark plug replacement varies slightly by model but typically ranges $250-350 for Outback and Forester. This includes the premium iridium or platinum plugs Subaru specifies, plus labor for the installation which is more involved on Subaru's boxer engines than on traditional inline engines. The horizontal cylinder configuration requires removing various components to access plugs, increasing labor time beyond what spark plug replacement requires in many other vehicles.

A 2018 Outback owner from Centerville had followed the 60,000-mile interval precisely, replacing spark plugs at 60K and again at 120K miles. At 120,000 miles when plugs were removed, the old plugs showed exactly the wear pattern Subaru's engineers predicted: electrodes eroded to the point where gap had increased approximately 0.010 inches beyond specification, and the center electrode showed slight rounding from erosion. The plugs were still functional but clearly at the end of their service life. This perfectly illustrated why the 60,000-mile interval is appropriate—it catches plugs just before wear becomes problematic.

Symptoms of Worn Spark Plugs

Several symptoms indicate your Outback or Forester's spark plugs have worn beyond optimal performance and need replacement. Recognizing these signs allows you to address the problem before misfires damage expensive catalytic converters.

Reduced fuel economy is often the earliest symptom of worn spark plugs. As plugs wear and gaps increase, the spark becomes less robust and doesn't ignite the air-fuel mixture as completely. Unburned fuel passes through the engine and eventually burns in the catalytic converter or exits as emissions. You might notice your typical 27-28 mpg highway efficiency dropping to 24-25 mpg over several months. This gradual decline makes it easy to miss unless you track fuel economy consistently.

Rough idle where the engine vibration seems more pronounced than normal indicates cylinders aren't firing consistently. Worn spark plugs create weaker sparks that may occasionally fail to ignite the mixture properly, causing occasional misfires at idle that you feel as increased vibration or roughness.

Hesitation during acceleration, particularly when merging onto highways or passing on two-lane roads, shows the engine isn't producing power smoothly across all cylinders. Worn plugs can't ignite the mixture as quickly or completely, reducing power output and creating the sensation that the engine is laboring or hesitating when you demand full power.

Check engine light illumination with misfire codes (P0300 series) indicates the engine computer has detected combustion problems. Modern Subarus monitor each cylinder's contribution to power output, and when spark plugs are worn enough to cause inconsistent combustion, the computer detects it and illuminates the warning light. Never ignore misfire codes, as continued driving with active misfires can damage catalytic converters.

Difficulty starting, especially in cold weather, suggests worn spark plugs struggling to ignite the mixture under challenging conditions. Cold engines require stronger sparks to ignite the denser air-fuel mixture, and worn plugs may not provide adequate spark for reliable starting.

Engine surging or jerking at steady cruise speeds indicates inconsistent combustion across cylinders. This symptom typically appears when plug wear has become quite severe and multiple cylinders are experiencing occasional misfires.

Increased emissions or failed emissions testing results from incomplete combustion due to worn spark plugs. Ohio's E-Check emissions testing in some counties will detect elevated hydrocarbon emissions caused by worn plugs, potentially resulting in test failure.

Reduced power during highway passing maneuvers shows the engine can't deliver full power when multiple cylinders are compromised by worn spark plugs. You might notice that passing acceleration that used to feel strong now feels merely adequate.

An Outback owner from Beavercreek had been noticing slightly reduced fuel economy for several months but dismissed it as probably due to winter fuel formulations or increased traffic. When his check engine light illuminated during his commute on I-675, he brought the vehicle in immediately. We found code P0302 (cylinder 2 misfire) and discovered his spark plugs had 73,000 miles on them, well past the 60,000-mile replacement interval. All four plugs showed excessive wear and one plug in cylinder 2 had eroded so severely that it was misfiring regularly. After spark plug replacement, not only did the misfire code clear, but his fuel economy improved from 24 mpg to 28 mpg—the reduction he'd been dismissing as normal was actually costing him 15% in fuel efficiency.

Why Subaru's Boxer Engine Design Affects Spark Plug Service

Subaru's horizontally-opposed boxer engine configuration creates unique considerations for spark plug access and replacement that owners should understand.

Horizontal cylinder orientation means spark plugs are accessed from the sides of the engine rather than from the top as in traditional inline or V-configuration engines. This requires removing components like air intake ducts, battery and battery tray, coolant reservoir, or other items blocking access to the spark plug wells.

Access difficulty varies by engine and model year. The naturally aspirated 2.5L engine typically offers relatively straightforward access, while the turbocharged 2.4L and 2.0L engines may require removing additional components including intercooler piping and turbo heat shields. This access complexity increases labor time and is why Subaru spark plug service costs more than similar service on many other vehicles.

Cylinder head design on boxer engines positions spark plug wells horizontally, which means any oil leaks from valve cover gaskets can allow oil to pool in the spark plug wells. If valve cover gaskets are leaking when spark plugs are replaced, the new spark plugs will be contaminated with oil, causing misfires and potentially damaging ignition coils. Reputable shops inspect for valve cover gasket leaks during spark plug service and recommend replacement if leaks are present.

Ignition coil configuration on modern Subarus uses coil-on-plug design where individual ignition coils sit directly atop each spark plug. These coils must be removed to access spark plugs, and their condition should be inspected during service. Ignition coils have finite lifespans (typically 80,000-120,000 miles) and often show signs of degradation when spark plugs are removed for service.

Torque specifications for spark plug installation are critical in boxer engines because the horizontal orientation means plugs thread into the sides of aluminum cylinder heads. Proper torque (typically 13-16 ft-lbs depending on plug type) ensures gas-tight sealing without overstressing the aluminum threads. Under-torquing can allow combustion leakage, while over-torquing can damage threads requiring expensive thread repair or cylinder head replacement.

The sequential replacement procedure matters because removing all four plugs simultaneously isn't recommended. Professional technicians typically replace plugs one at a time to prevent confusion about which coil goes with which cylinder and to minimize the time the combustion chambers are open to potential contamination.

OEM vs. Aftermarket Spark Plugs: Making the Right Choice

When replacing spark plugs in your Outback or Forester, you'll encounter options ranging from genuine Subaru OEM plugs to various aftermarket alternatives. Understanding the differences helps you make informed decisions balancing cost and quality.

Genuine Subaru OEM spark plugs are the exact plugs installed at the factory and are engineered specifically for your engine. These premium iridium plugs cost $15-25 each ($60-100 for a set of four) and are guaranteed to meet Subaru's performance specifications. OEM plugs are the safest choice, eliminating any question about compatibility or quality.

NGK and Denso OEM-equivalent plugs are manufactured by the same companies that supply Subaru's factory plugs and meet identical specifications. These plugs cost $12-20 each ($48-80 for a set) and provide essentially identical performance to genuine Subaru plugs. Many technicians consider these equivalent to OEM and use them routinely.

Premium aftermarket plugs from reputable manufacturers like Bosch, Champion, or Autolite range from $8-15 each. These plugs may use slightly different electrode materials or designs but are engineered to meet or exceed factory specifications for your specific engine. Quality varies by brand, and using premium brands from reputable manufacturers generally provides good service.

Budget plugs costing $3-6 each should be avoided. These typically use copper electrodes rather than the iridium or platinum materials Subaru specifies, meaning they won't last anywhere near the 60,000-mile interval. You'll save $30-50 on parts but need replacement at 15,000-30,000 miles, negating any savings while paying multiple times for labor.

Plug heat range must match your engine's specifications exactly. Using plugs with incorrect heat range can cause pre-ignition, detonation, or fouling problems regardless of the plug's quality or cost. Always verify that replacement plugs match the heat range specification for your specific engine and model year.

Gap specification should be pre-set from the factory for modern iridium plugs, but verification is prudent. The gap must be within 0.002 inches of specification, and attempting to gap iridium plugs risks damaging the fine center electrode. If gap verification shows improper gap, return the plug for replacement rather than attempting to adjust it.

For most Subaru owners, genuine OEM or NGK/Denso OEM-equivalent plugs represent the best value. The modest cost difference between premium plugs and budget options is insignificant when amortized over the 60,000-mile service interval, and the peace of mind knowing you're using proven components justifies any premium.

A Forester XT owner from Kettering had been using budget copper plugs from a discount auto parts chain, replacing them every 20,000 miles because they'd wear out and cause rough running. Over 100,000 miles, he'd paid for spark plug service five times at approximately $200 each (less expensive plugs but same labor), spending $1,000 total. If he'd used proper iridium plugs and replaced them at 60,000 miles, he'd have paid for service just twice at $285 each, spending $570 total—a savings of $430 while enjoying better performance throughout.

The Relationship Between Spark Plugs and Other Ignition Components

Spark plugs work as part of an integrated ignition system, and problems with related components can affect plug performance and longevity.

Ignition coils generate the high voltage required to create sparks at the plugs. Modern coil-on-plug systems use individual coils for each cylinder, positioned directly atop the spark plugs. These coils typically last 80,000-120,000 miles but can fail earlier if stressed by worn spark plugs requiring excessive voltage. When spark plugs are replaced, coils should be inspected for cracks, corrosion, or oil contamination indicating developing problems.

Coil replacement timing often coincides with the second spark plug service. If plugs are replaced at 60,000 miles per schedule, coils are likely healthy. At the 120,000-mile spark plug service, coils are approaching or exceeding their typical lifespan and should be replaced preemptively or at least inspected carefully for signs of deterioration.

Valve cover gaskets seal the valve covers to the cylinder heads, preventing oil leaks. Leaking valve cover gaskets allow oil to seep into spark plug wells, contaminating plugs and potentially damaging ignition coils. On boxer engines, oil pools in the horizontal spark plug wells rather than draining away. Valve cover gasket replacement during spark plug service adds $200-400 but prevents the oil contamination that causes premature plug and coil failure.

Ignition timing controlled by the engine computer affects how spark plug electrodes wear. Modern Subarus continuously adjust ignition timing based on sensor inputs, knock detection, and driving conditions. If timing control problems develop, spark plugs may show unusual wear patterns or accelerated erosion.

Air-fuel ratio directly affects combustion temperatures and spark plug wear. Rich fuel mixtures (too much fuel) can cause carbon fouling on spark plugs, while lean mixtures (too little fuel) increase combustion temperatures that accelerate electrode erosion. Proper fuel system maintenance including clean fuel injectors and mass airflow sensors helps spark plugs last their full service life.

DIY Spark Plug Replacement: Feasibility and Considerations

Spark plug replacement on Subaru boxer engines is more complex than many other vehicles, but mechanically inclined owners can perform the service with appropriate tools and knowledge.

Required tools include: socket set with magnetic spark plug sockets (typically 14mm or 16mm depending on engine), ratchet with extensions, torque wrench capable of 13-16 ft-lbs, gap gauge for verification, and dielectric grease for coil boots. Some engines require removing components like air boxes or coolant reservoirs, which may require additional basic hand tools.

Access procedures vary by engine but generally involve: disconnecting the battery negative terminal, removing engine covers and intake components as necessary, disconnecting electrical connectors from ignition coils, removing ignition coils from spark plug wells, removing spark plugs using appropriate socket and extensions, inspecting old plugs and spark plug wells for oil or coolant contamination, installing new plugs torqued to specification, applying dielectric grease to coil boots, and reinstalling coils and all removed components.

Proper torque specification is absolutely critical. Over-torquing spark plugs damages aluminum cylinder head threads, while under-torquing allows combustion leakage. Use a torque wrench set to the specification for your engine (typically 13-16 ft-lbs), not an impact gun or excessive force with a ratchet.

One-at-a-time replacement prevents confusion about coil positions and minimizes time combustion chambers are open to potential contamination. Complete one cylinder fully before moving to the next rather than removing all plugs simultaneously.

Inspection during removal includes examining old plugs for unusual wear patterns, checking spark plug wells for oil or coolant contamination, and inspecting ignition coils for cracks or damage. Unusual conditions warrant professional diagnosis before proceeding.

Time requirements for DIY spark plug replacement on Outback or Forester range from 1-3 hours depending on your mechanical experience and which engine you have. The first time takes longer as you familiarize yourself with component locations and removal procedures.

Professional service costs $250-350 including parts and labor, and includes warranty on work performed, expertise to identify related problems, proper torque specifications, and time savings. For many owners, the modest cost difference between DIY and professional service makes professional service the better value when you consider time, tools, and peace of mind.

Your 30-Day Spark Plug Maintenance Plan

This week: Check your Outback or Forester's service records to determine when spark plugs were last replaced and current vehicle mileage. Calculate how many miles remain until the 60,000-mile replacement interval (or 6-year time interval, whichever comes first). If records don't show spark plug replacement and your vehicle has exceeded 60,000 miles or 6 years, schedule service this week. While checking records, note your vehicle's current fuel economy by calculating mpg over several fill-ups to establish a baseline for monitoring future performance.

Within two weeks: If you're experiencing any symptoms of worn spark plugs (reduced fuel economy, rough idle, hesitation, check engine light), schedule diagnostic service immediately rather than waiting for your scheduled replacement interval. Have the shop scan for diagnostic codes and inspect spark plugs if symptoms suggest ignition problems. If your spark plug replacement is approaching within the next 10,000 miles, request that the technician inspect valve cover gaskets for leaks during the service. Leaking gaskets should be replaced simultaneously with spark plugs to prevent oil contamination of the new plugs.

By month's end: Add spark plug replacement to your long-term maintenance schedule at the appropriate intervals (60,000 miles or 6 years). Set calendar reminders at 55,000 miles and again at the 5-year mark so you have advance notice to budget for the service. If you're planning to keep your Outback or Forester beyond 120,000 miles, budget for ignition coil replacement during the second spark plug service, as coils typically need replacement at 80,000-120,000 miles. Create a simple maintenance log tracking major services including spark plug replacement, as this documentation increases resale value by proving proper maintenance history.

These steps take less than two hours total but ensure your Subaru's ignition system receives proper maintenance throughout your ownership, protecting performance, efficiency, and expensive emission control components.

Schedule Your Spark Plug Service Today

Spark plug replacement is straightforward preventive maintenance that provides immediate and measurable benefits in performance, fuel economy, and reliability. The difference between proper spark plug maintenance and neglecting this service can easily be $500-1,000 in wasted fuel, reduced performance, and potential catalytic converter damage over the life of your vehicle.

Our certified Subaru technicians understand the specific requirements of boxer engine spark plug service and use the correct OEM or OEM-equivalent plugs specified for your engine. Our service includes comprehensive inspection of ignition coils, valve cover gaskets, and related components to identify any issues requiring attention. We use proper torque specifications and procedures to ensure your spark plugs are installed correctly for optimal performance and longevity.

Schedule your spark plug replacement service today by calling our service department or booking online. We're located at 995 Miamisburg Centerville Road in Washington Township, easily accessible from I-675, Route 48, and Alex Bell Road. Don't wait until reduced performance, poor fuel economy, or check engine lights indicate your spark plugs are severely worn. Follow Subaru's recommended replacement interval and maintain the efficient, responsive performance your Outback or Forester was engineered to deliver.

Proper spark plug maintenance ensures optimal performance, maximizes fuel efficiency, protects expensive emission control components, and maintains the reliable operation that makes Subaru vehicles perfect for Ohio driving. That's the performance your Subaru deserves. ⚡🔧