The invention relates to a fault detection of a bypass diode in a variable voltage convert system. A vehicle power train includes a variable voltage converter, a bypass diode and a controller. The bypass diode may be coupled in parallel with the converter and configured to clamp an output of the converter to a battery voltage. The controller may be configured to, while in a boost mode and in response to a rate of change of a converter inductor current exceeding a boost threshold, open switches of the converter to divert propulsive energy through the bypass diode. The controller may also be configured to, while in buck mode and in response to a magnitude of a converter inductor current exceeding a buck threshold, open switches of the converter to divert regenerative energy through the bypass diode, while reversed biased, to the battery.