Spark plug

A spark plug code (NGK, Denso, Champion and others) describes the thread diameter, heat range, reach and electrode type. Because the wrong heat range or reach can damage the engine, use the code in the manual or the manufacturer's cross-reference chart.

What the plug colour tells you

• Light brown / grey: mixture and temperature are normal
• Black, dry soot: rich mixture, choke left on, clogged air filter
• Wet, oily: oil burning past the rings or valve stem seals
• White, blistered: lean mixture or overheating

Standard nickel plugs are usually replaced every 6,000–12,000 km; iridium and platinum plugs last longer. Check the electrode gap against the manual.