THINGS TO LOOK FOR
Ignition leads either deliver high voltage or they start to isolate and end up open circuiting thus creating a misfire.  This shouldn’t be ignored as a misfire may be causing damage to the exhaust system.  Also it may cause damage to the ignition coil as the high voltage can create a carbon track from the secondary lug to the primary lug destroying the coil.  A faulty lead may make the engine difficult to start.  All leads should be tested for resistance. 

IS IT EXPENSIVE TO REPAIR?
Ignition leads should be purchased in a set as they are replaced all at once.  When purchasing leads buy a well-known brand as even new leads can be faulty and testing is required before fitting to make sure that the leads are not faulty. 

TEMPORARY REPAIR AND LONG TERM REPLACEMENT
Testing or fitting any leads must be carried out one at a time as ignition leads are designed to fire in a firing order.  For this reason it is best to remove and replace one lead at a time to prevent messing up the firing order, thus creating a potential backfire.

This article is found in the Virtual mechanic CD Rom
You can download it for the price of a latte, but you will learn not to buy a lemon
By Darren Gow-Brown, Melbourne Australia ©2009