Diagnose faults where no prescribed process or format is available in light vehicle environments

Business Sectors (Suites): Maintenance and Repair - Light Vehicle
Developed by: IMI
Approved on: 30 Mar 2020


This NOS is about devising and implementing strategies to diagnose faults when the application of standard manufacturer diagnostic procedures has failed to reveal the source and cause of problems.  You are also required to identify the best course of action to be taken to correct problems.

Performance criteria

You must be able to:

P1 use suitable personal and vehicle protective equipment throughout all diagnostic related activities in the workshop
P2 confirm with the relevant people that all standard diagnostic procedures and techniques have been systematically and correctly applied to the vehicle prior to undertaking further work
P3 prepare the vehicle systems and work area for safe working procedures as appropriate to the vehicle
P4 analyse all previous system fault information, diagnostic test methods and results correctly to verify the inconclusive results prior to undertaking further work 
P5 liaise with the relevant manufacturer's representative to obtain up to date information, advice and guidance relevant to the identified fault, when necessary
P6 use diagnostic methods which are relevant to the symptoms presented
P7 collect diagnostic information in a logical, systematic and structured way which progressively eliminates all possible causes of the fault
P8 apply the checks and tests that are most likely to be effective in revealing the cause of the fault
P9 carry out all diagnostic activities following:

P9.1 your workplace procedures
P9.2 health, safety and environmental requirements
P9.3 environmental requirements
P10 work in a way which minimises the risk of:

P10.1 damage to other vehicle systems
P10.2 damage to other components and units
P10.3 contact with leakages
P10.4 contact with hazardous substances
P11 use any equipment required, correctly and safely throughout all diagnostic and rectification activities
P12 collect sufficient diagnostic information to enable an accurate diagnosis of the fault
P13 correctly identify the cause(s) of the fault
P14 identify and record any system deviation from acceptable limits accurately
P15 accurately ensure your assessment of dismantled sub-assemblies, components and units identifies their condition and suitability for repair or replacement
P16 make clear recommendations for a suitable course of action to rectify the fault
P17 inform the relevant person(s) promptly where repairs are uneconomic or unsatisfactory to perform
P18 complete all system checks and tests in the most cost and time effective way for the fault presented
P19 complete all system diagnostic activities within the agreed timescale
P20 ensure your records are accurate, complete and passed to the relevant person(s) within the agreed timescale and in the format required
P21 report any anticipated delays in completion to the relevant person(s) promptly

Knowledge and Understanding

You need to know and understand:

Legislative and organisational requirements and procedures
K1 the legislation and workplace procedures relevant to 

K1.1 health and safety
K1.2 the environment (including waste disposal)
K1.3 appropriate personal and vehicle protective equipment
K2 legal requirements relating to the vehicle (including road safety requirements)
K3 your workplace procedures for:

K3.1 recording fault location and correction activities
K3.2 reporting the results of tests
K3.3 the referral of problems
K3.4 reporting delays to the completion of work
K4 how to formulate and construct your own diagnostic procedures and processes in order for diagnostic activities to proceed
K5 the importance of documenting diagnostic and rectification information
K6 the importance of working to agreed timescales and keeping others informed of progress
K7 the relationship between time, costs and productivity
K8 the importance of reporting anticipated delays to the relevant person(s) promptly

Electrical and electronic principles

K9 electrical and electronic principles including types of sensors and actuators, their application and operation
K10 how electrical and electronic vehicle systems operate, including electrical component function, electrical inputs, outputs, voltages and oscilloscope patterns, digital and fibre optics principles
K11 the interaction between electrical, electronic, mechanical and hydraulic components and systems within a vehicle, including multiplexing
K12 electrical symbols, units and terms
K13 electrical safety procedures 
K14 the hazards associated with working on or near high energy electrical vehicle components

Use of diagnostic and rectification equipment *

K15 how to prepare and check the accuracy of *diagnostic testing equipment
K16 how to use diagnostic and rectification equipment for mechanical, electrical, hydraulic/pneumatic and fluid systems, specialist repair tools and general workshop equipment

Vehicle system faults, their diagnosis and correction

K17 how vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are constructed and operate
K18 how vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are dismantled, reassembled and adjusted to manufacturers’ specification
K19 the types and causes of vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid system, component and units faults and failures
K20 vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid component and units replacement procedures, the circumstances which will necessitate replacement and other possible courses of action
K21 how to find, interpret and use sources of information on vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid system operating specifications, diagnostic test procedures, repair procedures and legal requirements
K22 how to select the most appropriate diagnostic testing method for the symptoms presented
K23 how to carry out systematic diagnostic testing of vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems
K24 how to interpret, evaluate and analyse test results and vehicle data in order to identify the location and cause of vehicle system faults
K25 how to carry out the rectification activities in order to correct faults in vehicle mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems
K26 your workplace procedure, policy and procedure for:

K26.1 work carried out under warranty
K26.2 liaising with manufacturers and outside agencies
K27 the relationship between test methodology and the faults repaired – the use of appropriate testing methods
K28 how to make cost effective recommendations for rectification


*1. Causes of faults** are:

1.1. mechanical
1.2. electrical
1.3. electronic
1.4. hydraulic/pneumatic

2. Faults cover the:

2.1. vehicle engine area
2.2. transmission and driveline area
2.3. chassis system area
2.4. electrical and electronic units and components area

3. Diagnostic methods are:

3.1. measurement
3.2. functional testing
3.3. electrical and electronic systems testing

4. Diagnostic testing is defined as:

4.1. verify the fault
4.2. collect further information
4.3. evaluate the evidence
4.4. carry out further tests in a logical sequence
4.5. rectify the problem
4.6. check all systems

5. Equipment is:

5.1. diagnostic and rectification equipment for mechanical systems
5.2. diagnostic and rectification equipment for electrical and electronic systems
5.3. diagnostic and rectification equipment for hydraulic/pneumatic and fluid systems
5.4. specialist repair tools
5.5. general workshop equipment

6. Operating specifications include:

6.1. limits
6.2. fits
6.3. tolerances

Scope Performance

Scope Knowledge





This section contains examples and explanations of some of the terms used but does not form part of the standard.
Diagnostic equipment:
Examples include electronic testing equipment, brake testing equipment, steering geometry equipment, wheel balancing equipment, emission test equipment, measuring instruments, special service tools, charging service stations, noise and vibration detection equipment.

Examples include: dismantling, return to manufacturer, repair or replacement.

Rectification activities* are defined as:
A suitable repair, replacement, re-coding or re-programming that rectifies the fault(s) identified from the diagnostic activities carried out.

These can be any of the following types of light vehicles: SI, CI, Hybrid, Electric or Alternative fuel vehicles.

Alternative fuel:
This is defined as any type of fuel that may be used to power an internal combustion engine, examples would include LPG, bio ethanol etc.


Engine area:
Engines, cooling systems, electronic ignition, petrol fuel injection, diesel fuel injection, engine management systems, starting systems, charging systems.

Transmission and driveline area:
Clutch assemblies, clutch operating systems, manual gear boxes, automatic gear boxes (including electronic control), drivelines and hubs and final drive assemblies.

Chassis or Frame area:
Suspension systems, assisted steering systems, non-assisted steering systems, braking systems, ABS/traction control, wheels and tyres, stability systems, bodywork and related areas.

Electrical and electronic area:*
Body electrical systems (including wiring harnesses, lighting systems, auxiliaries, CANBUS systems, fibre optics, vehicle condition and monitoring, comfort and convenience, alarm systems), supplementary restraint systems (SRS), heating and air conditioning systems, climate control, communication equipment, navigation systems and entertainment equipment.

Links To Other NOS

External Links

Version Number


Indicative Review Date

31 Mar 2024





Originating Organisation


Original URN


Relevant Occupations

Engineering, Light Vehicle Diagnostic Technician (Automotive), Light Vehicle Fleet/Service Manager (Automotive), Light Vehicle Master Technician (Automotive), Vehicle Trades

SOC Code



Diagnose; faults; prescribed; process; format; motor; vehicle; guided; diagnosis