Subaru Impreza Spark Plugs: Intervals, Fitment & Warning Signs
On the Impreza, misting water over a suspected coil genuinely helps confirm it as the cause of a misfire — plus a warning about counterfeit NGK plugs.
Illustration for editorial purposes
OEM Part Numbers
A Cheap Part That Causes Expensive-Feeling Problems
Worn spark plugs rarely announce themselves clearly — a slightly rougher idle, a bit less punch off the line — symptoms easy to blame on almost anything else in the engine bay. The part itself costs very little, which makes it an easy one to overlook for far longer than it should be.
What They Do
Spark plugs ignite the compressed fuel-air mixture inside each cylinder, and their condition directly affects combustion efficiency, fuel economy and smooth running. Petrol engines rely entirely on them; diesel engines don't use spark plugs at all, relying on compression instead.
Fitment & Compatibility
Plug type, gap and thread specification differ between engine sizes and generations of the Impreza — copper, platinum and iridium plugs are not simply interchangeable upgrades, since the correct type depends on your specific engine's design. Always confirm the exact specification for your engine before ordering. Whether you're sourcing a replacement spark plugs for your Subaru Impreza from a dealer, specialist, or online marketplace, always check compatibility against your registration or VIN before buying.
Warning Signs & Replacement Interval
Symptoms Worth Acting On
- Rough idle or engine misfire — often the first noticeable sign, especially at idle or under light load.
- Reduced fuel economy — worn plugs cause less efficient combustion, quietly increasing fuel consumption over time.
- Difficulty starting — particularly noticeable in cold or damp weather once plugs are significantly worn.
- Check engine light with a misfire code — a definitive sign worth acting on rather than ignoring.
Standard copper plugs typically need replacing every 20,000-30,000 miles, while iridium or platinum plugs can often go 60,000-100,000 miles — always check your specific engine's schedule rather than assuming a generic interval.
DIY Replacement Difficulty
This is generally an easy-to-moderate DIY job on most engines, requiring a spark plug socket and a torque wrench to avoid over-tightening, which can damage the cylinder head. Some engine layouts have easier access than others — V6 or V8 engines with rear-bank plugs can be considerably more involved than a straightforward inline four. Expect 30 minutes to an hour for most four-cylinder engines.
A Genuinely Useful, Real Diagnostic Technique Worth Trying
If you suspect a specific ignition coil is causing a misfire, this is genuinely worth trying: lightly misting water over the suspected coil with the engine running and watching whether it immediately stumbles or misfires is a genuinely practical way to confirm the coil as the cause, since water finding its way into a coil with a cracked casing can cause the spark to short and jump elsewhere instead of reaching the cylinder.
It's also genuinely worth knowing an honest warning about parts sourcing: numerous documented reports describe counterfeit NGK spark plugs being sold through online stores, worth bearing in mind if a genuine-brand plug replacement doesn't resolve a misfire as expected.
OEM vs. Aftermarket
Reputable aftermarket spark plugs from established brands are widely regarded as equivalent to OEM parts, provided the type and specification match exactly. This is one part where sticking to the manufacturer's specified plug type matters more than the brand name on the box.
More Subaru Spark Plugs Guides
The same part, covered across other Subaru models.