Make a continuity test between both wires to make sure there is not a break in either wire(earth cut out wire, and earth return wire) if they are good, with the engine stopped take a new wire from the ignition cut out terminal on the ignition unit, and earth it out, remove the spark plug and test for a spark, if it still sparks a new ignition unit is reqired, if it does not spark there is an issue with the stop switch.
Husqvarna man, January 2010
