In the majority of cases the inspection above will reveal
the cause and hence the action required to resolve the fault.
There will be occasions when this inspection will reveal nothing
and the fault cannot be reproduced to order. This is usually
the result of an intermittent fault or a fault whose cause
is external to the system. These are the most difficult faults
to resolve and require the investigation to taken further.
Assume that the fault exists until it has been established
that the occurrence was an isolated incident and that the
system is functioning correctly.
At this stage in the investigation process time should be
taken to consider the results obtained so far. It may be necessary
to recheck some findings and repeat some tests.
Look at the environment in which the system is installed.
Pay attention to the physical layout, nearby machinery or
equipment which may be having an effect on the system or its
components. Consider each possibility in isolation and in
conjunction with the others. Nearby will include
adjacent buildings and possibly the premises of neighbouring
It will also be useful to seek the advice of colleagues and
question the customer further regarding the reported fault
and its circumstances.
Examine the possible causes that have been identified and
asses each for its effect on the system. In the case of equipment
this should be switched on and used in its normal manner to
determine if there is any detrimental effect to the system.
If there are multiple pieces of machinery or equipment listed
as possible causes then try varying combinations.
If the precise source cannot be determined then prioritise
the candidates in order from most probable to least probable,
leaving out any that have been eliminated.
Should this lead to a specific source of the fault being identified
then work can begin to eliminate its effect. With an intermittent
equipment fault the affected units can be replaced or if the
source is external then shielding and screening may be required.
If the cause is still not apparent, but the fault persists
then the equipment should be assumed suspect and substituted
piece by piece in an attempt to remove, by elimination, the
Before starting, use the fault report to list by probability,
all the connected equipment. Use this list to swap out the
most likely candidates first. Since this will usually involve
the system to be powered down for central items consideration
must be shown to the customer and a convenient time decided
upon. Should there only be a single opportunity to power down
the system then all components which need this to be swapped
must be swapped together. Bear in mind that this may affect
the accuracy of the final diagnosis.
If the entire system as been replaced and the fault is still
present the cause is almost certainly external and the list
of probable causes will need re-assessment.
When a fault defies the investigation, check with the equipment
manufacturer to see if they are aware of the type of fault
and its possible causes. Do they have it under investigation
? Do they have a solution ? What advice can they give regarding
possible causes ?
The engineering staff will have experienced a wide range of
problems and it is wise to seek their advice as this can save
a great deal of time and money.
If the fault is new then the manufacturer will usually begin
their own investigation.