The Fault Follows the Operator, Not the Equipment: Decision Tree

Why this matters

Some faults are attached to a person, not a part. The equipment works perfectly for one user and fails for another, on the same unit, the same day. That is not a random intermittent, it is a technique-dependent fault, and it means the variable you are hunting is a human action, not a component. Miss that, and you will swap good parts forever while the complaint tracks whoever is using the machine. This tree isolates the operator as the variable and finds the specific thing they do differently.

Start here: is anyone at risk from how it's being operated?

Before treating this as a benign technique quirk, check whether the operator's action creates a hazard: overriding a safety, forcing a jammed mechanism, running equipment in an unsafe way.

If the technique is dangerous - defeating an interlock, forcing a stuck part, operating with a guard removed, creating a shock, scald, or overpressure risk - stop and address the hazard first: shut the equipment down, correct the unsafe condition, and make the safe method non-optional (restore the guard or interlock) before you continue diagnosing. A technique problem that is also a safety problem is a safety problem first.

If it is not a safety issue, isolate the operator below.

Confirm the fault actually follows the person

Before you chase technique, prove the pattern:

• Have the complaining user operate it while you watch. Does it fail for them?
• Operate it yourself the same way. Does it work for you?
• If it fails for them and works for you on the identical unit, the operator is confirmed as the variable. If it fails for both, this is not a technique fault, so go back to the equipment.

Watch them do it, don't ask them to describe it

People cannot reliably describe their own routine. They tell you the version they think is correct, not the one their hands actually perform. So watch, do not interview:

• Say nothing and let them run it start to finish, exactly as they normally would.
• Look for the delta: the order they do steps, how hard or fast, how long they wait, which control they touch, what they skip.

The difference between their sequence and yours is the fault. It is almost always something they do not know they are doing.

Name the technique delta

Common operator variables across trades:

• Timing - not waiting for a cycle, purge, or fill to finish before the next step.
• Force or speed - forcing a valve, slamming a door, cranking past a stop.
• Sequence - doing steps in an order that defeats an interlock or starves a stage.
• Partial operation - not fully seating, closing, latching, or engaging a control.
• Frequency - operating so often the equipment never recovers between cycles.

Match what you saw to one of these and you have the diagnosis.

Fix the technique, not the machine

The repair here is often not a part at all:

• Correct the specific action, and have them do it right in front of you until it works.
• Where the equipment lets a person do the wrong thing easily, consider whether a control change, a label, or a mechanical stop makes the right way the easy way. That is a legitimate fix, not a workaround.
• If the technique that triggers the fault is genuinely reasonable use, then the equipment is too fragile for its job, and now you do have an equipment conversation: the right part, setting, or upgrade so normal use stops breaking it.

Recap the order

1. Safety first. If the technique is dangerous, correct the hazard and make the safe method non-optional before diagnosing.
2. Confirm the fault follows the person: they fail, you don't, same unit.
3. Watch them operate it silently; do not rely on their description.
4. Name the technique delta: timing, force, sequence, partial operation, frequency.
5. Fix the action, and make the right method the easy one; escalate to equipment only if normal use is what breaks it.

When the fault tracks the hand on the controls, the hand is the diagnosis.

References

• Human-factors and manufacturer operating guidance on correct equipment operation
• Trade-standard practice: observe actual operation before attributing or replacing components
• See related: Separating a Usage Habit From an Equipment Fault; You Can't Reproduce What the Customer Does (decision tree)