The safety interlocks typically don't act alone. The seat switch is typically in series with the PTO and blade engagement switch. In this way you can't get off the seat when the blade is engaged. I highly suggest you...
• Get a electrical schematic for your machine.
• Use an Ohmmeter to test (repeatedly) each of the switches after disconnecting one (1) of the attached leads.
After 5 years any of those switches could become intermittent.... especially if you occasionally wash the mower.