Was your trimmer registered to you when your purchased it or do you have proof of purchase? If so and you have a bad coil it should fall in the limited lifetime replacement. Before march of 2003 and back to a certain date that for the life of me I can not remember ignition coils were and are covered under a limited life time replacement, the limited end of the warranty was for the first 2 years or normal warranty period it was parts and labor,after 2 years it was parts only for the life of the unit and for the registered owner. Before this time frame units carried a lifetime warranty parts and labor. After march 2003 its parts and labor for 2 years and parts 5 years . These warranties only cover the coil/ignition module.
Try the unit and see if you have spark, if none,disconnect the kill wires from the coil if you have spark its the kill circuit,no spark its coil. no need for meters etc.