Ignition Leads and Spark Plug Wires

Ignition leads (also called spark plug wires or HT leads) carry the high-voltage charge from the ignition coil or distributor cap to the spark plug. The spark plug then uses that charge to create the spark that ignites the air-fuel mixture inside the combustion chamber. They are separate components - leads can fail independently from plugs and vice versa. Coil-on-plug (COP) engines do not use ignition leads as each coil sits directly on its spark plug.

Common signs of worn or failing leads include visible arcing or sparking from the lead insulation in the dark, misfires that move between cylinders when you swap leads, and cracked or oil-soaked insulation visible on inspection. Worn spark plugs typically cause hard starting, rough idle, misfires under load, and increased fuel consumption. An OBD2 scanner showing cylinder-specific misfire codes will help identify which cylinder is affected, guiding diagnosis of both components.

Spark plug service intervals vary by plug type - standard copper plugs typically last 30,000 to 40,000 km, while iridium and platinum plugs can last 80,000 to 100,000 km depending on the vehicle. Ignition leads generally last 60,000 to 100,000 km but should be inspected at every major service for cracking or damage. Both components should be replaced together when one has worn to service limit, as the labour to access them is often the same.

It is best practice to replace all spark plugs as a set. Plugs wear at similar rates and mixing old and new plugs creates inconsistent combustion across cylinders. Ignition leads should also be replaced as a set - individual leads are difficult to source for most vehicles and worn leads from the same set will fail soon after. The cost difference between replacing one and replacing all is small compared to the labour of a repeat service.

Yes. Misfires caused by worn plugs or failing leads send unburnt fuel into the exhaust system. This unburnt fuel overheats the catalytic converter and can cause permanent damage - a costly secondary repair. Extended misfiring can also foul oxygen sensors and contaminate engine oil with fuel wash-down. Replacing plugs and leads at their service interval is significantly cheaper than replacing a catalytic converter.

Most aftermarket OEM replacement spark plugs are pre-gapped to the standard specification for the intended application and do not require adjustment. However always verify the gap against your vehicle service manual using a feeler gauge before installation - particularly on iridium and platinum plugs where the electrode is fragile and must not be adjusted with a coin or flat tool. If the gap is incorrect use a proper spark plug gap tool and adjust only the ground electrode, never the centre electrode.