Locate the mark
NGK explains that corona staining can form on the exposed ceramic above the metal shell as electrical fields attract particles. That external brown band is different from inspecting the firing end inside the combustion chamber. A photograph needs location context.
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Look for actual damage
Cracked ceramic, a damaged boot, carbon tracking or an abnormal firing end deserve separate inspection. Do not label every brown mark as combustion gas escaping through the plug. Keep removed plugs identified by cylinder.
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Use symptoms as evidence
If the engine misfires, record codes, load and temperature conditions and assess the whole ignition circuit. A cosmetic stain alone is not a reason to buy upgraded coils. Use the correct replacement schedule and application rather than the color of one external mark.
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Sources / references 1
- Manufacturer technical reference ↗Reviewed Oct 6, 2026
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