john deere 345 lawn tractor

[mmrepa]

Engine kills when brake is released, seat switch is functioning properly

 

[pile81]

Did you check the seat switch with a multi meter, just because the switch plunger is moving dosent mean the switch is working internally. ...

 

[BBO]

Engine kills when brake is released, seat switch is functioning properly

These safety features on mowers seem to drive folks nuts. Brake switch, seat switch, reverse logic etc...
Some switch gets flaky and it's hard to find what the wiring logic is doing.

Some seat switches you can simply disconnect. Some you have to install a jumper wire.
Brake pedal may have to be pushed in to crank it.
Reverse may disengage the blades, on and on.

Lawyers will all disagree, but I need to mow forward and reverse without pushing a button or pulling out a knob.
If I need to get off to pick up a stick I may not choose to wear out the clutch by turning it off...

Find the owners manual and see what the 345 logic is, and this should help to see what the problem is.
I suspect the brake or seat switch, but that's only a weak guess...