Diagnosis Fundamentals · The Diagnostic Method
The Six Diagnostic Categories
Supply, Control, Transmission, Components, User/Application, Environmental — diagnostic categories, not a rigid sequence.
These six categories give you a complete mental map of where a fault can live. They are categories, not a rigid troubleshooting sequence. You choose your starting point from the complaint and observation, then use the categories to make sure you haven't missed a whole area of the system.
Credit where it's due: these six categories were drilled into me by Neal Kidman — the best refrigeration teacher in Australia. Once you learn them, they stick like shit to a return air filter.
1. Supply
Does the required supply actually reach the equipment? For electrical components that is correct voltage at the terminals. For the refrigeration circuit that is the correct refrigerant charge and liquid supply to the metering device. No supply, no operation — regardless of how good the rest of the system is.
2. Control
Is the control system actually calling for the equipment to run, and are the safeties allowing it? Thermostats, pressure switches, oil safety, defrost controls, sensors and controllers. A healthy component that isn't being told to run won't run.
3. Transmission
Is the energy or substance getting from where it is to where it needs to be? Wiring and connections, pipework, ductwork, shafts, couplings, liquid lines. A break or restriction in transmission looks like a component failure until you trace the path.
4. Components
Is a component itself faulty? Compressor, condenser, evaporator, metering device, fans, pumps, valves, contactors, capacitors. This is where most techs start — it should usually be one of the last places you conclude, after supply, control and transmission are proven.
5. User / Application Factors
Is the system still matched to the application it was designed for? Increased production, more product, warmer product, more door openings, longer hours, changed setpoints. Not every system that can't hold temperature has a refrigeration fault — sometimes the load has outgrown the system.
6. Environmental Factors
Has the environment changed around the system? Ambient, condenser airflow, solar exposure, insulation, room construction, seasonal changes. Environmental conditions plus application load equal refrigeration demand.
Field Rules
- 01
"The six categories are a map, not a sequence. Choose your starting point from the evidence."
- 02
"Most "component failures" are supply, control or transmission faults that haven't been traced yet."
Diagnosis at a glance
Common mistakes & callbacks
- •Jumping straight to Components and skipping Supply, Control and Transmission.
- •Treating User/Application and Environmental factors as excuses instead of measuring them.
