A method of testing an electronic unit by comparing resulting signal shapes from the unit to be tested and a known functioning unit. The method includes powering off the units for testing and feeding one or more predefined signal shapes of two or more different frequencies as input signals to the known functioning unit and to the unit to be tested at corresponding test points. The method further includes measuring the resulting signal shapes from both units at corresponding measurement points and comparing at least one resulting signal shape from the known functioning unit with the corresponding resulting signal shape from the unit to be tested. The method also includes detecting a fault in the unit to be tested on the basis of an existing signal shape distortion in time axis of the resulting signal shape received from the unit to be tested.