Why does the key refuse to turn after changing the switch on a riding lawn mower?
If you are using the key that came with the new ignition switch, there shoud be no reason for the key not to turn unless the switch was installed upside down which would mean the key would have to turn in the opposite direction. If the switch was installed correcly, try using a little graphite lube (a really fine-powdered graphite that comes in a squeezable tube) into the switch cylinder to loosen it up. Good luck.
