Fault Diagnosis Tests

Fault Diagnosis tests are used to select technical personnel who need to be able to find and repair faults in electronic and mechanical systems. As modern equipment of all types becomes increasingly complex and more dependent on electronic control systems the ability to approach problems logically in order to find the cause of the fault is increasingly important.


This type of test usually forms part of a test battery in which verbal aptitude and numerical aptitude are also tested.

Fault Diagnosis for Military Jobs
As military hardware becomes more sophisticated and dependant on electronic control systems it demands higher standards of technical and maintenance personnel. The ability to approach a maintenance or repair problem in a logical and systematic way to to locate the fault is critical when working with complicated systems like those found in aircraft, ships and communications equipment.

Example Fault Diagnosis Question
There are two components to this question.

  1. A grid which describes the function of a series of switches.

  2. A flowchart showing: input – switches – output.

Each of the switches acts to alter the input before the next switch in the series.


This grid shows four switches and their effect on four numbered inputs which may be either on or off. The first switch (diamond symbol) inverts inputs one and two. That is, if the input is on, it is turned off and if it is off it is turned on. The other switches act in a similar way as detailed in the grid. If a switch is not working then it has no effect on the input which passes through unchanged.


In this example flowchart, all of the inputs (numbered 1, 2, 3, 4) are on.
When these inputs pass through the first switch 1 & 3 are inverted (i.e. switched off).
These modified inputs (1=off, 2=on, 3=off, 4=on) then pass through the second switch.
The second switch inverts inputs 3 & 4, which gives (1=off, 2=on, 3=on, 4=off).
These modified inputs (1=off, 2=on, 3=on, 4=off) then pass through the third switch.
The third switch should invert inputs 2 & 4, giving (1=off, 2=off, 3=on, 4=on).

However the third switch is not working so the output from switch two is not changed.
The answer to this question is therefore ‘C’ – because switch type ‘C’ is at fault.

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