My Fisher and Paykel Aerotech OS302 oven will not heat up to desired temperature.

This type of problem normally can be ascribed to a defective thermostat. Unfortunately there is no test for a thermostat, it has to be replaced to see if the problem has been sorted out.
One thing you can do is remove the thermostat bulb from the oven and place place it in a position where it will not heat up. Place an oven thermometer in the oven. Switch on and see of the temperature climbs above what you wee seeing before. This is a very much hands on test as you have no control over the temperature that the elements can get to. As soon as you see the temperature reach a point above that seen before, shut down immediately.