A power electronics unit for an aircraft and a method of determining the integrity of a power electronics unit are provided. The power electronics unit comprises an electric machine controller; a heatsink arranged to conduct heat from the electric machine controller; a housing comprising a sealed internal volume enclosing the electric machine controller and heatsink; a dielectric liquid partially filling the internal volume to cover the electric machine controller; a gas pocket within the internal volume; a pressure sensor arranged to measure a pressure of gas within the gas pocket; a temperature sensor arranged to measure a temperature within the internal volume; and a controller. The controller is configured to receive signals from the pressure sensor and temperature sensor, determine a pressure and temperature from the received signals and provide an output signal dependent on the determined temperature and pressure.