Why Do They Fail?

Although often overlooked, HT leads are a common cause of ignition issues. They can degrade due to:

• Heat and age, leading to insulation cracking or hardening

• Internal resistance increase, reducing voltage at the spark plug

• Oil contamination, especially around the rocker cover

• Poor terminal connections, from loose fitting or corrosion

• Mechanical damage, such as crushing or abrasion

• Incorrect routing, causing arcing or crossfire

Symptoms of Failing Ignition Leads

• Engine misfires, especially under load or in damp conditions

• Rough idle, loss of power or hesitation when accelerating

• Check Engine Light (MIL) with misfire-related codes

• Increased fuel consumption due to poor combustion

• Visible arcing or “ticking” noises near the lead path

• Worn spark plugs, as they compensate for weak spark delivery

Fitting Tips

1. Replace leads as a full set — mixing old and new can mask faults

2. Follow OE routing paths to avoid heat or interference

3. Push connectors firmly onto coil and plug until you feel a ‘click’

4. Avoid pulling on the cable when removing — use the boot

5. Test resistance with a multimeter — OE-spec leads should be under 20kΩ per metre

✅ Unsure whether it’s the lead, plug, or coil? Call TechASSIST for guided diagnostics and fitting advice.