Sample Fault Diagnosis Question

Fault Diagnosis questions 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 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.


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.


This grid shows four switches and their effect on four coloured inputs which may be either on or off. For example, the first switch (arrow symbol) allows all colours to pass when working but when at fault allows only red to pass.


In this example, all of the inputs (Red, Yellow, Green, Blue) are on.

When these inputs pass through the first switch, all colours are allowed to pass if the switch is working and only red is allowed to pass if the switch is at fault.

This modified input passes through the second switch which allows only red and yellow to pass if the switch is working and only green and blue to pass if the switch is at fault.. 

This modified input passes through the third switch which turns on red and green if the switch is working and turns on yellow and blue if the switch is at fault.

Once again, you need to compare the input and output and determine which of the switches is at fault. In this case, it is switch ‘A’ (the first switch) which allows all colours to pass when working but allows only red to pass when at fault.

Note that the fourth switch (not used in this question) ‘toggles’ the lights. That is, it switches a light off if it is on and vice versa.

Download a sample Fault Diagnosis Test

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

You may also be interested in:

Mechanical Aptitude Tests, Levers, Pulleys, Gears, Springs, Electricity, Tools and Shop Arithmetic.

Pass Mechanical Reasoning