The Fault That Clears the Moment You Arrive

Why this matters

You get the call, you drive out, and the system runs perfectly the second you walk in. The customer feels foolish, you feel stuck, and the easy write-up is "no fault found." But a fault that clears the moment you arrive is often telling you something specific: the act of getting to it changed the very condition that caused it. That is different from an ordinary intermittent that comes and goes on its own. Here, your arrival is the variable. Learn the handful of ways that happens and you stop losing these calls.

This is not the same as an intermittent

An intermittent fault comes and goes on its own schedule whether or not you are there. An arrival-cleared fault is correlated with you: something about the drive out, the wait, the power-down, or the customer's prep changed the state. The distinction matters because the fix is different. For a true intermittent you instrument and wait. For an arrival-cleared fault, you work backward from what your arrival changed. Confusing the two sends you on a stakeout for a fault that only your own hands will ever trigger.

The five ways arrival clears a fault

Almost every one of these calls fits one of five mechanisms. Name which before you guess.

• Thermal reset. The fault needs heat or cold that has since passed. It failed at the peak of the afternoon, or on the first cold start, and by the time you arrive the temperature has moved. Overheating connections, marginal capacitors, and expansion-driven leaks all do this.
• Power interruption. The customer, or you, cycled the power to "show you," and the reset cleared a latched fault or a hung controller. The system is fine now because it was rebooted, not because it was repaired.
• Reseated by handling. Someone opened a panel, jiggled a plug, or moved the unit to give you access, and a loose or corroded connection made contact again. The access itself was the temporary fix.
• Load or time dependent. The fault only appears under a condition absent right now: a full house of water draws, a peak electrical load, a specific time of day, a duty cycle you are not running. It is not there because the trigger is not there.
• Customer cleanup. They reset the breaker, cleared the error, changed the filter, or "tidied up" before you came, erasing the state you needed to see.

Work backward from what changed

Once you suspect an arrival-cleared fault, interview for the delta between the failure and now:

• "What is different right now versus when it failed?" (temperature, load, who touched it)
• "Did anyone reset, power-cycle, open, or clean anything before I got here?"
• "What time does it happen, and what else is running then?"

Then try to recreate the failing condition deliberately: heat-soak the component, restore the load, put the panel back and run it closed. If you can bring the fault back by restoring the condition, you have found it.

When you cannot recreate it on the spot

Do not sign "no fault found" when you have a live lead. Two honest closes:

• Instrument for the condition, not just for time: a min/max clamp reading across the suspected load window, a temperature logger on the hot component, a pressure gauge left in place.
• Bound it and set a trigger: tell the customer exactly what you confirmed, what condition you could not recreate, and to call while it is still failing and leave everything as-is, especially not to power-cycle it.

"Leave it broken and call me" is the single most useful thing you can say on these calls, because the customer's instinct to reset is what erased the evidence last time.

References

• Trade-standard diagnostic practice on intermittent and condition-dependent faults
• Manufacturer documentation on thermal and load-related fault behavior
• See related: When the Act of Inspecting Disturbs the Fault, Capturing an Intermittent Fault