Diagnose and rectify heavy goods and public service vehicle combustion engine and component faults

Business Sectors (Suites): Maintenance and Repair - Heavy Goods and Public Service Vehicles
Developed by: IMI
Approved on: 30 Mar 2021


This standard is about diagnosing and rectifying faults occurring in heavy goods and public service vehicle combustion engine mechanical, electrical, hydraulic/pneumatic 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, vehicle systems and work area for safe working procedures as appropriate to the vehicle and environment 
P4 prepare, connect and check all the required equipment following manufacturers' instructions prior to use
P5 use diagnostic methods which are relevant to the symptoms presented
P6 collect sufficient diagnostic information in a systematic way to enable an accurate diagnosis of engine system faults
P7 identify and record any system deviation from manufacturer’s tolerances 
P8 ensure your assessment of dismantled sub-assemblies, units and components identifies their condition and suitability for repair or replacement
P9 promptly inform the relevant person(s) where repairs are uneconomic or unsatisfactory to perform 
P10 use the equipment required, correctly and safely throughout all rectification activities
P11 carry out all rectification activities following:

P11.1 manufacturers' instructions
P11.2 industry recognised repair methods
P11.3 your workplace procedures
P11.4 health, safety and environmental requirements
P12 work in a way which minimises the risk of:

P12.1 damage to other vehicle systems, units and components
P12.2 contact with leakages and hazardous substances
P12.3 damage to your working environment
P12.4 injury to self and others
P13 ensure all repaired and replaced units and components conform to the vehicle operating specification and any legal requirements
P14 adjust units and components correctly to ensure that they operate to meet system requirements, when necessary
P15 promptly record and report any additional faults you notice during the course of work 
P16 use testing methods which are suitable for assessing the performance of the rectified system 
P17 ensure the rectified combustion engine system performs to the vehicle operating specification and any legal requirements prior to return to the customer/driver
P18 ensure your records are accurate, complete and passed to the relevant person(s) within the agreed timescales in the format required
P19 complete all system diagnostic activities within the agreed timescale
P20 promptly report any anticipated delays in completion to the relevant person(s)

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 the implications on an Operators Licence of not carrying out repairs and inspections correctly
K3 legal requirements relating to the vehicle (including road safety requirements and European Emissions Standards )
K4 your workplace procedures for

K4.1 recording diagnostic and rectification activities
K4.2 the referral of problems
K4.3 reporting delays to the completion of work
K5 the importance of working to industry recognised diagnostic and rectification procedures and processes and obtaining the correct information for diagnostic and rectification activities to proceed
K6 the importance of recording diagnostic and rectification information
K7 the importance of working to agreed timescales and keeping others informed of progress
K8 the relationship between time, costs and productivity
K9 the importance of promptly reporting anticipated delays to the relevant person(s) 

Electrical and electronic principles
K10 electrical and electronic principles associated with engine systems, including types of sensors and actuators, their application and operation
K11 how electrical and electronic engine systems operate, including electrical component function, electrical inputs, outputs, voltages and oscilloscope patterns, digital and fibre optics principles
K12 the interaction between electrical, electronic and mechanical components with vehicle engine systems
K13 how engine electrical systems interlink and interact, including multiplexing
K14 electrical symbols, units and terms
K15 electrical safety procedures
K16 the hazards associated with working on or near high voltage electrical vehicle components

Use of diagnostic and rectification equipment
K17 how to prepare and check the accuracy of diagnostic testing *equipment
K18 how to use diagnostic and rectification equipment for combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems (including any emissions controls); specialist engine repair tools and general workshop equipment

Combustion engine electrical faults, their diagnosis and rectification
K19 how combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are constructed, operate, dismantled and reassembled
K20 how combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems are dismantled, reassembled and adjusted to manufacturers’ specifications
K21 the types and causes of combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid system, unit and component faults and failures
K22 combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid unit and component replacement procedures, the circumstances which will necessitate replacement and other possible courses of action
K23 how to find, interpret and use sources of information on combustion engine electrical and electronic operating specifications, diagnostic test procedures, repair procedures and legal requirements
K24 vehicle operating specifications for limits, fits and tolerances relating to combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems for the types of vehicle on which you work
K25 how to select the most appropriate diagnostic testing method for the symptoms presented
K26 how to carry out systematic diagnostic testing of combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems using a prescribed process or format and diagnostic methods *
K27 how to assess the condition of mechanical, electrical, electronic, hydraulic/pneumatic and fluid units and components
K28 how to interpret test results and vehicle data in order to identify the location and cause of vehicle system *faults

K29 how to carry out rectification activities in order to correct faults in the combustion engine mechanical, electrical, electronic, hydraulic/pneumatic and fluid systems
K30 the relationship between test methodology and the faults repaired – the use of appropriate testing methods
K31 how to make cost effective recommendations for rectification
K32 the importance of inspecting the vehicle following any repairs


1. Faults occur within:

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

2. Diagnostic methods are:

2.1. sensory
2.2. measurement
2.3. functional testing
2.4. 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 combustion engine mechanical systems
4.2. diagnostic and rectification equipment for combustion engine electrical systems
4.3. diagnostic and rectification equipment for combustion engine hydraulic/pneumatic and fluid systems
4.4. specialist repair tools
4.5. general workshop equipment

5. Rectification activities are:

5.1. dismantling
5.2. replacement of units and components
5.3. adjustment of units and components
5.4. repairs to wiring and connectors
5.5. re-programming vehicle systems
5.6. reassembly
5.7. functional testing

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 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.

Heavy goods and public service vehicles
These are medium and large goods vehicles, buses and coaches of 3500kgs gross vehicle mass (GVM) and above.

Examples include: servicing, dismantling for further inspection and test, repair and replacement.

Sensory testing methods
These may include looking, listening, smelling and touching for heat.

Links To Other NOS

External Links

Version Number


Indicative Review Date

31 Mar 2024





Originating Organisation


Original URN


Relevant Occupations

Engineering, Heavy Vehicle Diagnostic Technician (Automotive), Vehicle Trades, Bus and Coach Mechanic, Bus and Coach Mechelec

SOC Code



Engine; units; components; heavy goods vehicle; public service vehicle; diagnose; rectify; electrical; hydraulic; mechanical; bus; coach