parts – the knowledge of the system and the abil-

Diagnostic techniques
7
Double check the fault has been rectified and
that you have not caused any further problems.
The stages of faultfinding will be used exten-
sively to illustrate how a logical process can be
applied to any situation.
2.2.2 The art of diagnostics
The knowledge needed for accurate diagnostics
is in two parts:
1. understanding of the system in which the
problem exists;
2. having the ability to apply a logical diagnostic
routine.
The knowledge requirement and use of diagnostic
skills can now be illustrated with a very simple
example. After connecting a hose pipe and turning
on the tap, no water comes out of the end! Your
knowledge of this system tells you that water
should come out providing the tap is on, because
the pressure from a tap pushes water through the
pipe, and so on. This is where your diagnostic
skills become essential. The following stages are
now required.
1. Confirm that no water is coming out by look-
ing down the end of the pipe!
2. Does water come out of the other taps, or did
it come out of this tap before you connected
the hose?
3. Consider what this information tells you; for
example, if the answer is ‘Yes’ the hose must
be blocked or kinked.
4. Walk the length of the pipe looking for a kink.
5. Straighten out the hose.
6. Check that water now comes out and that no
other problems have been created.
Much simplified I accept, but the procedure you
have just followed made the hose work and it is
also guaranteed to find a fault in any system. It is
easy to see how it works in connection with a hose
pipe and I’m sure anybody could have found that
fault (well most people anyway)! The higher skill
is to be able to apply the same logical routine to
more complex situations. The routine (Table 2.1)
is represented by Figure 2.1. The loop will con-
tinue until the fault is located. I will now explain
each of these steps further in relation to a more
realistic automotive workshop situation – not that
getting the hose to work is not important!
Often electrical faults are considered to be the
most difficult to diagnose – but this is not true. 
I will use a vehicle cooling system fault as an
example here, but electrical systems will be 
covered in detail in later chapters. Remember that
the diagnostic procedure can be applied to any
problem, mechanical, electrical or even personal!
However, let’s assume that the reported fault
with the vehicle is overheating. As is quite com-
mon in many workshop situations that’s all the
information we have to start with. Now work
through the six stages.
●
Stage 1
Take a quick look to check for obvi-
ous problems such as leaks, broken drive belts
or lack of coolant. Run the vehicle and con-
firm that the fault exists. It could be the tem-
perature gauge for example.
●
Stage 2
Is the driver available to give more
information? For example, does the engine
overheat all the time or just when working
hard? Check records, if available, of previous
work done to the vehicle.
●
Stage 3
Consider what you now know. Does
this allow you to narrow down what the cause
of the fault could be? For example, if the vehicle
Verify the fault
Collect further
information
Evaluate the
evidence
No
Carry out further
tests in a logical
sequence
Yes
Rectify the fault
Check all systems
Fault located?

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