A repeatable method protects you from panic, tunnel vision, and the urge to change parts at random.
SURVIVAL RULE: Find the first missing condition.
Define the symptom precisely
‘It does not work’ is not a fault description. Ask what command was given, what should have happened, what actually happened, when it last worked, whether anything changed, whether it fails every time, and what alarms or physical signs appear. Precision at the start prevents hours of random work.
‘The infeed conveyor starts, but stops after two seconds when a box reaches sensor PE3’ is already a diagnostic clue. It identifies the sequence point and repeatability.
Field habit: Repeat the symptom in safe conditions if permitted, and write one sentence that another technician could reproduce.
Common trap: Do not let the loudest person define the fault with a theory such as ‘the PLC is bad.’ Keep symptom and theory separate.
Find the first missing condition
Machines operate as sequences. Each step depends on conditions from the previous step. Instead of asking ‘what part is broken?’ ask ‘what is the first thing that should be true but is not?’ This question converts a large system into a boundary.
If a cylinder should extend after a photoeye detects a part, check: photoeye changes, PLC input changes, logic requests extend, output changes, valve actuates, air is present, cylinder moves. The first failed transition tells you where to investigate.
Field habit: Use the HMI, PLC status, LEDs, meter readings, pressure indicators, and physical observation to walk the chain.
Common trap: Do not skip ahead to the component you are most comfortable replacing.
Quick check
What should be true at this point in the sequence?
What evidence can prove or eliminate this section of the system?
What changed recently or only under the failing condition?
Divide the system
When the problem is broad, split it into meaningful halves: control versus power, electrical versus mechanical, upstream versus downstream, command versus feedback, field versus PLC, machine versus external utility. Each test should eliminate a large area.
If a motor contactor is energized and correct three-phase voltage is present at the motor terminals but the shaft does not turn, the control system is no longer your main search area. The problem is motor or mechanical load.
Field habit: Prefer tests with high information value: one safe measurement that rules out half the system is better than five random checks.
Common trap: Do not disassemble a mechanism until you have evidence the problem is mechanical.
Change one thing, then verify
Part swapping destroys evidence when several components are changed together. Once you have a proven fault, make one controlled repair, restore the machine correctly, then test multiple cycles and operating modes. A single successful cycle is weak verification.
After replacing a sensor, confirm alignment, input state, sequence timing, cable routing, and several product cycles. Then document the failed device and why you believe it failed.
Field habit: Keep removed parts labeled until the repair is verified. This helps if the fault returns.
Common trap: Do not declare victory because the machine moved once.
Quick check
What should be true at this point in the sequence?
What evidence can prove or eliminate this section of the system?
What changed recently or only under the failing condition?
Field Exercise
During a real fault, deliberately identify the first missing expected condition before replacing any component.
PART II
CORE ELECTRICAL AND CONTROLS SKILLS