Subaru Spark Plug Replacement Cost (Boxer Engine)
Replacement of spark plugs on Subaru H4 (4-cylinder) or H6 (6-cylinder) Boxer engines. Due to the horizontal layout, workspace is extremely tight against the frame rails.
Why this fails
Standard electrode wear occurs over 60k-100k miles (depending on model year); FB20/FB25 and FA20/FA24 engines are high-compression or turbocharged, making precise spark timing critical for preventing pre-detonation.
What the job involves
- Removal of the intake airbox assembly and the battery/washer fluid reservoir to gain access to the cylinder heads.
- Disconnecting and extracting the ignition coils from the narrow gap between the engine and the frame rail.
- Using a combination of 'wobble' extensions and spark plug sockets to extract the old plugs in limited space.
- Hand-threading new Iridium plugs to prevent cross-threading the aluminum heads.
- Torquing plugs to spec (approx 12-15 ft-lb) and reinstalling ignition coils with a small amount of dielectric grease.
If you delay this repair
Delayed replacement leads to ignition coil failure (an extra $400+ repair), reduced fuel economy, carbon buildup on valves, and potential catalytic converter damage due to unburnt fuel (misfires).
Save money
- • Purchase NGK Laser Iridium plugs from a reputable auto parts store instead of the dealer parts counter; they are the exact same plug for roughly $5-$10 less per unit.
- • Replace the engine air filter at the same time since the airbox must be removed anyway, saving you double labor later.
- • Avoid 'Laser Platinum' or generic copper plugs; while cheaper, they will trigger knock sensors and fail significantly earlier in high-heat Boxer engines.
OEM part numbers
FAQ
How much does subaru spark plug replacement cost (boxer engine) cost in 2026?
Expect $240–$650 at most US shops, parts plus labor.
How long does the job take?
1.5–3.5 labor hours typically.
Can I do this myself?
Yes — DIY difficulty 4/5.
What happens if I delay?
Delayed replacement leads to ignition coil failure (an extra $400+ repair), reduced fuel economy, carbon buildup on valves, and potential catalytic converter damage due to unburnt fuel (misfires).