Diagnose and rectify light vehicle combustion engine and component faults

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


This NOS is about diagnosing and rectifying faults occurring in the vehicle combustion engine's mechanical, electrical, hydraulic and fluid systems.

Performance criteria

You must be able to:

P1 use suitable personal and vehicle protective equipment when using diagnostic methods and carrying out rectification activities
P2 support the identification of faults, by reviewing vehicle:

P2.1 technical data
P2.2 diagnostic test procedures
P3 prepare the vehicle systems and work area for safe working procedures as appropriate to the vehicle
P4 prepare, check and use all the required equipment following manufacturers' instructions 
P5 use diagnostic methods which are relevant to the symptoms presented
P6 collect sufficient diagnostic information in a logical and systematic way to enable an accurate diagnosis of engine system faults
P7 identify and record any system deviation from manufacturer’s tolerances accurately
P8 ensure your assessment of dismantled sub-assemblies, components and units identifies their condition and suitability for repair or replacement 
P9 inform the relevant person(s) promptly where repairs are uneconomic or unsatisfactory to perform
P10 carry out all diagnostic and rectification activities following:

P10.1 manufacturers' instructions
P10.2 recognised repair methods
P10.3 your workplace procedures
P10.4 health, safety and environmental requirements

P11 work in a way which minimises the risk of:

P11.1 damage to other vehicle systems
P11.2 damage to other components and units
P11.3 contact with leakages

P11.4 contact with hazardous substances
P12 ensure all repaired and replacement components and units conform to the vehicle operating specification and any legal requirements
P13 adjust components and units, when necessary, correctly to ensure that they operate to meet system requirements
P14 record and report any additional faults you notice during the course of work promptly
P15 use testing methods which are suitable for assessing the performance of the system rectified

P16 ensure the rectified engine system performs to the vehicle operating specification and any legal requirements prior to return to the customer
P17 ensure your records are accurate, complete and passed to the relevant person(s) within the agreed timescale and in the format required
P18 complete all system diagnostic activities within the agreed timescale
P19 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 the importance of working to recognised diagnostic and rectification procedures and processes and obtaining the correct information for diagnostic and rectification activities to proceed
K5 the importance of recording 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 associated with engine systems, including types of sensors and actuators, their application and operation
K10 how electrical and electronic engine systems operate, including electrical component function, electrical inputs, outputs, voltages and oscilloscope patterns, digital and fibre optics principles
K11 the interaction between electrical, electronic and mechanical components within vehicle engine systems
K12 how engine electrical systems interlink and interact, including multiplexing
K13 electrical symbols, unit and terms
K14 electrical safety procedures 
K15 the hazards associated with working on or near high energy electrical vehicle components

Use of diagnostic and rectification equipment 

K16 how to prepare and check the accuracy of *diagnostic testing equipment
K17 how to use diagnostic and rectification equipment for engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems; specialist engine repair tools and general workshop equipment

Combustion engine electrical faults, their diagnosis and rectification *

K18 how engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are constructed and operate
K19 how engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are dismantled, reassembled and adjusted to manufacturers’ specifications
K20 the types and causes of engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid system, component and unit *faults and failures
K21 engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid component unit and replacement procedures, the circumstances which will necessitate replacement and other possible courses of action
K22 how to find, interpret and use sources of information on engine electrical and electronic operating specifications, diagnostic test procedures, repair procedures and legal requirements
K23 vehicle operating specifications for manufacturer’s tolerances relating to engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems for the vehicle(s) on which you work
K24 how to select the most appropriate diagnostic testing method for the symptoms presented
K25 how to carry out systematic diagnostic testing of engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems using prescribed processes or formats
K26 how to assess the condition of mechanical, electrical, electronic, hydraulic/pneumatic and fluid components and units
K27 how to interpret test results and vehicle data in order to identify the location and cause of engine system faults

K28 how to carry out the rectification activities in order to correct faults in the engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems

K29 the relationship between test methodology and the faults repaired – the use of appropriate testing methods
K30 how to make cost effective recommendations for rectification


1. Faults* occur within:

1.1. the engine mechanical system
1.2. the engine electrical and electronic systems 
1.3. the engine hydraulic and fluid systems

2. Diagnostic methods are:

2.1. measurement
2.2. functional testing
2.3. electrical and electronic systems testing

3. Diagnostic testing is defined as:

3.1. Verify the fault
3.2. Collect further information
3.3. Evaluate the evidence
3.4. Carry out further tests in a logical sequence
3.5. Rectify the problem
3.6. Check all systems

4. Equipment is:

4.1. diagnostic and rectification equipment for engine mechanical systems
4.2. diagnostic and rectification equipment for engine electrical systems
4.3. diagnostic and rectification equipment for engine hydraulic/pneumatic and fluid systems
4.4. specialist repair tools
4.5. general workshop equipment

Scope Performance

Scope Knowledge





This section contains examples and explanations of some of the terms used but does not form part of the standard.

Agreed timescales:
Examples include: manufacturer's recommended work times, job times set by your company or a job time agreed with a specific customer.

Diagnostic information:
This relates to mechanical condition, including wear, run out, pressures and compressions, flow, leakage and electrical measurements such as voltage and pulse displays, electronic systems data, including fault codes, sensor measurements and control unit outputs and/or signals.

Engine area:
Engine mechanical, cooling systems, electronic ignition, petrol fuel injection, diesel fuel injection, lubrication, engine management systems, exhaust emission reduction systems and pressure charged induction systems and starting/charging.

Engine and component faults:
These are faults that require a multistage inspection and a series of test results to identify the cause.

Functional testing:
Examples include: compression testing, performance testing and road testing where relevant.

Hydraulic and fluid systems:
These are fuels, oil, lubrication, cooling, air conditioning.

Examples include: servicing, dismantling for further inspection and test, repair and 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 light vehicles: Additionally, these vehicles may be SI, CI, Hybrid 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.

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), Vehicle Trades

SOC Code



Diagnose; rectify; motor; vehicle; combustion engine; components; faults; light; electronic; electrical; hydraulic; mechanical