Fault Diagnosis Tests

Fault Diagnosis tests usually form part of a test battery in which verbal reasoning and numerical reasoning and mechanical reasoning also feature. They are used to select personnel for technical roles who need to be able to find and repair faults in electronic control systems. The questions in fault diagnosis tests appear rather abstract and require pure logic to solve. This is because when a fault develops in an electronically controlled system, there is often no physical clue as to the cause. In the absence of such a clue, for example an obviously burned-out component, the only way to diagnose the problem is by a process of logical elimination.

Fault Diagnosis

This type of test is used extensively to select technical and maintenance personnel as well as to select for artificer (technical) roles within the armed forces where the ability to approach problems logically in order to find the cause of the fault is increasingly important.

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.

Fault Diagnosis Example

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.


Diamond Symbol Series

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.

This sample question paper contains 12 questions and has a suggested time limit of 10 minutes. The questions are presented in Letter/A4 format for easy printing and self-marking.

Download Fault Diagnosis Test - Practice Test 1

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