Pico coil on plug extension leads pp399 pico pp399 coil on plug extension leads are the simple accessory that allows you to take accurate secondary ignition measurements on coil on plug ignition systems they have been designed to aid diagnostics on secondary ignition circuits by allowing an ht measurement to be made when there are no or limited access to any spark plug leads the leads are specifically designed for use with the pp178 secondary ignition pickupthe leads are easy to use sim