How old is the truck, around late 2009 / early 2010 build the sensor spec changed.

Has it got the old mechanical type sensors OR the newer proximity type sensors?

There was an issue with the proximity sensors fitted on the early build of trucks at the changeover point where the platforms wasnt shimmed correctly at the factory, the arm needs to be within 6.5mm of the sensor area to activate the sensor.

The old type mechanical switches the ends with the sensor wheel can get smashed off.

The is 1 switch on the safety arms.

There are 2 switches on the platform.

1 switch senses when the platform is folded UP for walk along mode.
The 2nd switch senses when there is a person standing on the platform.

If any of the platfrom switches fail the truck wont drive as the built in safety logic wont allow it to.

The new proximity switches also suffer from people breaking the locking tab of the electrical connector plugs so the plug can then come loose.

Disconnect both platform sensors, fold the safety arms DOWN, walk the truck & lift the arms UP, truck should STOP - this will proove the arm switch is working correctly.