Keep the plugs in order
Label each removed plug by cylinder and photograph the firing end in consistent light. Record mileage, plug number and whether the engine had idled extensively before removal. One isolated image can miss the difference between a cylinder-specific problem and a condition affecting the whole engine.
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Appearance is a clue
NGK distinguishes dry and wet fouling, deposits and wear. A black or wet plug does not by itself identify the failed component. Fueling, ignition, oil entry and operating conditions can produce overlapping observations.
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Build the next test
Compare codes, coil and boot condition, compression information where appropriate and service history. Do not clean away all evidence before diagnosis. Correct the underlying cause before assuming a fresh set will permanently resolve the problem.
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Sources / references 1
- Manufacturer technical reference ↗Reviewed Oct 6, 2026
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