OEM & aftermarket ignition cable from top brands — compare prices, check fitment, order with free delivery
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Ignition Cable buying guide — what to know before you buy
Spark plug leads (HT leads) carry high-voltage ignition pulses from the coil pack to the spark plugs. They are insulated with silicone or EPDM rubber to prevent voltage leakage. Degraded leads cause misfires, poor fuel economy, and increased emissions. They should be replaced as a set during ignition servicing.
Lead failure symptoms
HT leads degrade from heat, vibration, and age. The insulation breaks down, allowing voltage to leak to earth instead of reaching the spark plug.
Engine misfires, especially in damp conditions or at idle
Rough idle and reduced power
Visible sparking or arcing from the leads in dark conditions
Radio interference from electromagnetic radiation
Check engine light with misfire codes
Lead replacement
Replace leads as a complete set and route them carefully to avoid cross-firing between adjacent cylinders.
Replace one lead at a time to maintain the correct firing order
Route leads in their original clips and channels — crossing can cause misfires
Apply dielectric grease to the boots for moisture protection and easy future removal
Check lead resistance with a multimeter — compare to specification
Frequently Asked Questions
How do I know if my spark plug leads are bad?
Misfires in damp weather, rough idle, and visible arcing in the dark are common signs. Measure each lead's resistance — it should be within the manufacturer's specification.
Should I replace leads with spark plugs?
If the leads are more than 60,000 miles old, yes. Fresh leads ensure the new plugs receive full spark energy for optimal combustion.
Why do my leads arc in the rain?
Moisture on degraded insulation provides a conductive path to earth. The spark takes the path of least resistance — through the wet insulation rather than across the plug gap.
Are all spark plug leads the same length?
No. Each lead is a specific length for its cylinder. Leads are numbered or colour-coded. Mixing them up changes the firing order, causing the engine to run very poorly or not at all.
