The high voltage finding an easier path to earth than the plug gap - and it shows as a misfire that is worse in damp weather.

As insulation ages it hardens, cracks and becomes contaminated with oil and dirt.

The voltage then arcs along the outside of the lead, through a cracked boot, or across a distributor cap rather than jumping the plug gap.

Moisture provides a conductive path, which is why the symptom appears or worsens in rain and early morning damp.

Running the engine in the dark makes it visible directly: blue arcing along a lead or around a boot is conclusive.

A fine spray of water from a plant mister on a running engine will provoke it if the fault is intermittent - though this must be done carefully and away from anything electronic.

They can be, and the test is genuinely useful here - resistance rises as a carbon core degrades, and manufacturers publish a figure per unit length.

Measure end to end with the lead disconnected at both ends, contacting the terminals rather than the boots.

Compare against the specification for the lead's length, since a longer lead correctly reads higher.

An open circuit or a wildly high reading is conclusively failed.

A reading that changes as the lead is flexed indicates a broken core, which is a common and intermittent fault.

Resistance testing does not detect tracking, though - a lead can measure perfectly and still arc to earth through cracked insulation.

Carbon-impregnated cores are cheap and degrade with age; wound conductors over a magnetic core suppress effectively at low resistance and last much longer.

Both exist to suppress the radio-frequency interference an ignition spark generates.

A carbon core achieves it with high resistance along its length, and that carbon breaks down over time so resistance climbs.

A wound conductor uses inductance rather than resistance to suppress, so it can carry the spark energy with far less loss.

Wound leads generally last longer and deliver more energy to the plug, at higher cost.

Solid-core leads without suppression exist and should not be used on any road vehicle - they disrupt the vehicle's own electronics and are non-compliant in most markets.

Leads must stay clear of heat and moving parts, and parallel leads can cross-fire by inducing a spark in each other.

The separators and clips fitted by the manufacturer hold each lead in a defined position for a reason.

A lead resting on an exhaust manifold burns through quickly and arcs to earth.

Two leads for cylinders adjacent in the firing order, run parallel and touching, can induce a voltage in each other and fire the wrong cylinder.

That produces a misfire that moves around and defeats straightforward diagnosis.

Photograph the routing before removing an old set, and use the separators supplied rather than bundling the leads together.

Yes - they have aged identically, and the labour is in the access rather than the part.

All the leads on an engine have had the same heat, vibration and time.

One failed lead means the others are close behind, and a mixed set gives different resistance to different cylinders.

Sets are supplied with the correct lengths for each cylinder, which matters for routing and for avoiding strain.

Fit them one at a time, transferring from the old set, so no lead ends up on the wrong cylinder.

Replace the distributor cap and rotor at the same time where fitted, since tracking inside a cap gives identical symptoms and they age together.

By the boot, with a twist to break the seal - never by pulling on the lead itself.

The boot grips the plug or the cap terminal tightly and often bonds to it with age and heat.

Pulling on the lead separates the conductor from its terminal inside the boot, which destroys the lead and can leave the terminal behind.

Grip the boot, twist to break the seal, then pull straight off.

Boot pliers designed for the job make this much easier in confined spaces and are inexpensive.

A small amount of dielectric grease inside the boot on refitting makes the next removal far easier and helps seal against moisture.

On new vehicles largely yes - coil-on-plug has removed them - but they remain across older vehicles, small engines and plant.

Coil-on-plug puts a coil directly on each plug, so there is no high-tension lead at all.

That removes tracking, resistance loss and cross-firing as failure modes entirely, which is why the industry moved.

Older road vehicles, generators, agricultural and horticultural equipment, small engines and a great deal of plant still use leads and will for a long time.

Parts availability remains good, and quality sets are widely made for common engines.

For a fleet with mixed and older equipment, keeping a set for the highest-volume engines on the shelf turns a damp-morning misfire into a quick fix.