Quick test, remove the black wire from the ignition unit which joins into the wiring loom at the side of the engine, this will then bypass all micro switches, be aware while doing this twst you will not be able to kill the engine with the ignition switch.
Husqvarna man
May 2010
Most likely the seat switch is faulty.
phil_saunders(a)bigfoot.com
Phil Saunders
May 2010
I can answer this question...
riding mower engine dies when you engage the blades or release the clutch?
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Try fixfault.com to fix lawn mowers.