There are two things that influence the temperature of a dishwasher.
1. Heating element
2. Thermostat
The heating element is a coil which is responsible for turning the water hot that comes in to your dishwasher. While the thermostat controls the temperature of the heating element, how hot you want it to be on each cycle.
Using a multimeter you can check these components for resistance and replacing them is not that expensive too.