The engine fault exception is a a very broad rule and is broken when the engine releases a diagnostic fault code. You will need to open an exception report and examine the exception info column to determine why the rule was broken.
For example, you may get an engine fault exception and see engine coolant in the exception info. As a result, there was an engine fault exception in this particular case because the vehicle's engine had release a code saying that there was an issue with the vehicle's coolant which then triggered an engine fault exception.
Below is a list of some examples of descriptions in the exception info column that you may see:
Device has been unplugged
Engine Oil Pressure
Engine Oil Temperature
General vehicle warning light